An avalanche photo diode provided with a guard ring around a photo detecting region having a pn junction, in which semiconductor layers of the photo detecting region having a pn junction, different in conductivity type from a semiconductor of the guard ring, are composed of a second semiconductor layer formed in contact with a first semiconductor layer forming the pn junction of the photo detecting region and of the same conductivity type as the semiconductor of the guard ring, and a third semiconductor layer formed in contact with the second semiconductor layer and having a larger band gap than the second semiconductor layer, and in which the tip end of the guard ring is formed to extend down into the third semiconductor layer. An avalanche photo diode can be formed to be provided with a uniformly thick, first semiconductor layer forming a photo detecting region and a second semiconductor layer forming a first pn junction between it and the first semiconductor layer, in which third and fourth semiconductor layers of the same composition as each other, respectively having larger band gaps than those of the first and second semiconductor layers, are provided to form therebetween a second pn junction which extends from the first pn junction to surround the peripheral portion of the first semiconductor layer.