We have developed a diamond bipolar transistor with a current amplification ratio of around 10 under the condition of the common-emitter configuration. The transistor consists of heavily phosphorus doped n-type and heavily boron doped p-type diamond layers in addition to well-controlled n-type and intrinsic diamond layers. This success has been achieved by the introduction of the low resistive n-type layer and the low contact resistance between metal and the n-type layer.