An avalanche diode includes a first semiconductor region of a first conductivity type disposed in a first depth, a second semiconductor region disposed in a second depth deeper than the first depth with respect to a first surface, in contact with the first semiconductor region, and a third semiconductor region disposed in a third depth deeper than the second depth with respect to the first surface, in contact with the second semiconductor region. Avalanche multiplication is caused by the first and third semiconductor regions. The first, second, and third semiconductor regions overlap in plan view. A potential difference between the first and second semiconductor regions with respect to main charge carriers of a semiconductor region of the first conductive type is smaller than a potential difference between the first and third semiconductor regions with respect to the charge carriers.