diff --git a/lib/embulk/output/bigquery.rb b/lib/embulk/output/bigquery.rb
index 44e8b79..a5dd966 100644
--- a/lib/embulk/output/bigquery.rb
+++ b/lib/embulk/output/bigquery.rb
@@ -244,7 +244,7 @@ def self.configure(config, schema, task_count)
           if task['range_partitioning']
             raise ConfigError.new "Partition decorators(`#{task['table']}`) don't support `range_partition`"
           end
-          task['time_partitioning'] = {'type' => 'DAY'}
+          task['time_partitioning'] ||= {'type' => 'DAY'}
         end
 
         if task['range_partitioning']