We measured leakage current and gain characteristics of a reverse-type avalanche photodiode (APD). The leakage current generated below the breakdown voltage was found to be lower than 1 pA at temperatures below 200 K. Avalanche multiplication of the APD was achieved in the temperature range from 150 mK to 300 K. To characterize charge carrier properties of the APD, output signal pulses from the APD were observed by irradiating the APD with X-rays in the temperature range from 150 mK to 4.2 K. The yield of signal charge was found to abruptly change in the temperature range from 1 to 2 K, where the yield of charge at 1 K is about 50% of that at 2 K.

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