An avalanche photodiode includes: a first semiconductor layer of a first conductivity type formed on a substrate of the first conductivity type; a second semiconductor layer of a second conductivity type formed under the first semiconductor layer; a third semiconductor layer of the first conductivity type formed in a shallow portion of the first semiconductor layer on the substrate, the third semiconductor layer having a higher concentration than an impurity concentration of the first semiconductor layer; a fourth semiconductor layer of the first conductivity type formed in a region in the first semiconductor layer immediately below the third semiconductor layer; a first contact electrically connected to the first semiconductor layer; and a second contact electrically connected to the second semiconductor layer. An impurity concentration of the fourth semiconductor layer is higher than that of the first semiconductor layer and is lower than that of the third semiconductor layer.