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We report the performance improvement of the silicon avalanche photodiode (APD) for electron detection by annealing. It was found that the dark current is reduced and that the gain and energy resolution are improved by annealing at 500 K for 10 h. By means of the nuclear reaction analysis, the depth distribution of hydrogen in APD was measured before and after annealing. The hydrogen concentration in the near-surface region was significantly increased by annealing. We discuss that passivation of the impurity and/or defect levels by hydrogen atoms is a possible reason for the performance improvement of the photodiode.