This paper proposes on-line based a deterioration diagnosis device for diagnosis pole transformers using tan$delta$ and FFT(fast Fourier transform). We measured tan$delta$ and temperature to diagnose pole transformer insulating oil, diagnostic results are processed by FFT. For measuring convenience, we use R/F(radio frequency) wireless data communication module operating by secondary voltage of pole transformer. We have voltage variation test and oil temperature variation test to prove usability of proposed diagnosis device. The result of this paper shows that the proposed device can be used as deterioration diagnosis device of pole transformers.