diff --git a/lib/ougai/formatters/base.rb b/lib/ougai/formatters/base.rb
index 1672b8f..40551f5 100644
--- a/lib/ougai/formatters/base.rb
+++ b/lib/ougai/formatters/base.rb
@@ -3,8 +3,13 @@
 
 module Ougai
   module Formatters
+    # Base formatter
+    # @attr [Fixnum] trace_indent Specify exception backtrace indent (by default this is 2).
+    # @attr [Fixnum] trace_max_lines Keep exception backtrace lines (by default this is 100).
+    # @attr [Boolean] serialize_backtrace Whether exception should converts String (by default this is on).
     class Base < Logger::Formatter
       attr_accessor :trace_indent, :trace_max_lines
+      attr_accessor :serialize_backtrace
       attr_reader :app_name, :hostname
 
       def initialize(app_name = nil, hostname = nil)
@@ -12,6 +17,7 @@ def initialize(app_name = nil, hostname = nil)
         @hostname = hostname || Socket.gethostname.force_encoding('UTF-8')
         @trace_indent = 2
         @trace_max_lines = 100
+        @serialize_backtrace = true
         self.datetime_format = nil
       end
 
@@ -25,14 +31,15 @@ def serialize_exc(ex)
           message: ex.to_s
         }
         if ex.backtrace
-          err[:stack] = serialize_trace(ex.backtrace)
+          bt = ex.backtrace.slice(0, @trace_max_lines)
+          err[:stack] = @serialize_backtrace ? serialize_trace(bt) : bt
         end
         err
       end
 
       def serialize_trace(trace)
         sp = "\n" + ' ' * @trace_indent
-        trace.slice(0, @trace_max_lines).join(sp)
+        trace.join(sp)
       end
 
       private
diff --git a/lib/ougai/formatters/readable.rb b/lib/ougai/formatters/readable.rb
index 887e542..db26b65 100644
--- a/lib/ougai/formatters/readable.rb
+++ b/lib/ougai/formatters/readable.rb
@@ -14,6 +14,7 @@ def initialize(opts = {})
         @trace_indent = opts[:trace_indent] || 4
         @plain = opts[:plain] || false
         @excluded_fields = opts[:excluded_fields] || []
+        @serialize_backtrace = true
         load_dependent
       end
 
@@ -28,6 +29,10 @@ def call(severity, time, progname, data)
         format_log_parts(dt, level, msg, err_str, data_str)
       end
 
+      def serialize_backtrace=(value)
+        raise RuntimeError, 'Not support serialize_backtrace'
+      end
+
       protected
 
       def format_log_parts(datetime, level, msg, err, data)
@@ -61,7 +66,7 @@ def create_err_str(data)
         return nil unless data.key?(:err)
         err = data.delete(:err)
         err_str = "  #{err[:name]} (#{err[:message]}):"
-        err_str += "\n    " + err[:stack] if err.key?(:stack)
+        err_str += "\n" + (" " * @trace_indent) + err[:stack] if err.key?(:stack)
         err_str
       end
 
diff --git a/lib/ougai/version.rb b/lib/ougai/version.rb
index d63d747..b55e694 100644
--- a/lib/ougai/version.rb
+++ b/lib/ougai/version.rb
@@ -1,3 +1,3 @@
 module Ougai
-  VERSION = "1.5.6"
+  VERSION = "1.5.7"
 end
diff --git a/spec/formatters/base_spec.rb b/spec/formatters/base_spec.rb
index 3685ae4..4f8c9f1 100644
--- a/spec/formatters/base_spec.rb
+++ b/spec/formatters/base_spec.rb
@@ -64,4 +64,35 @@
       expect(subject.hostname).to eq('myhost')
     end
   end
+
+  describe '#serialize_exc' do
+    let (:app_name) { 'myapp' }
+    let (:hostname) { 'myhost' }
+    let (:errmsg) { 'dummy error' }
+
+    it 'returning data with stack as String' do
+      begin
+        raise errmsg
+      rescue => e
+        result = subject.serialize_exc(e)
+      end        
+      expect(result[:message]).to eq(errmsg)
+      expect(result[:stack]).to be_instance_of(String)
+    end
+
+    context 'not serialize backtrace' do
+      it 'returning data with stack as Array' do
+        subject.serialize_backtrace = false
+        subject.trace_max_lines = 6
+        begin
+          raise errmsg
+        rescue => e
+          result = subject.serialize_exc(e)
+        end        
+        expect(result[:message]).to eq(errmsg)
+        expect(result[:stack]).to be_instance_of(Array)
+        expect(result[:stack].size).to eq(6)
+      end
+    end
+  end
 end