An avalanche photodiode according to the present invention includes, inside a substrate semiconductor layer having a first conductivity type and a uniform impurity concentration, a first semiconductor layer having the first conductivity type, a second semiconductor layer having a second conductivity type, a third semiconductor layer having the second conductivity type, a fourth semiconductor layer having the second conductivity type, a fifth semiconductor layer having the first conductivity type and formed at a position away from the third semiconductor layer in a lateral direction, a sixth semiconductor layer having the second conductivity type, a first contact, and a second contact. The first semiconductor layer is positioned just under the second semiconductor layer and the fifth semiconductor layer in contact therewith. An avalanche phenomenon is caused at a junction between the first semiconductor layer and the second semiconductor layer.