A SiPM photodetector, comprising: a silicon layer provided on a first main surface of a semiconductor substrate and of a first conductivity type; a first semiconductor layer provided in the silicon layer, of a first conductivity type, and having an impurity concentration higher than an impurity concentration of the silicon layer; a second semiconductor layer provided on the first semiconductor layer, of a second conductivity type, and forming a pn boundary with the first semiconductor layer; a third semiconductor layer provided in the silicon layer, of a first conductivity type, having an impurity concentration higher than that of the silicon layer, separated from the first semiconductor layer; a first electrode provided under a second main surface of the semiconductor substrate, and electrically connected to the silicon layer; and a second electrode provided above the second semiconductor layer, electrically connected to the second semiconductor layer, and not electrically connected to the third semiconductor layer; wherein the third semiconductor layer is disposed between the first semiconductor layer and the semiconductor substrate; the third semiconductor layer is peripheral such that, when viewed from above, it surrounds the first semiconductor layer, and an end portion of the third semiconductor layer overlaps with an end portion of the first semiconductor layer, or wherein the third semiconductor layer surrounds the first semiconductor layer with no overlap between the third semiconductor layer and the first semiconductor layer; and the third semiconductor layer is not electrically connected to the first electrode. a silicon layer provided on a first main surface of a semiconductor substrate and of a first conductivity type; a first semiconductor layer provided in the silicon layer, of a first conductivity type, and having an impurity concentration higher than an impurity concentration of the silicon layer; a second semiconductor layer provided on the first semiconductor layer, of a second conductivity type, and forming a pn boundary with the first semiconductor layer; a third semiconductor layer provided in the silicon layer, of a first conductivity type, having an impurity concentration higher than that of the silicon layer, separated from the first semiconductor layer; a first electrode provided under a second main surface of the semiconductor substrate, and electrically connected to the silicon layer; and a second electrode provided above the second semiconductor layer, electrically connected to the second semiconductor layer, and not electrically connected to the third semiconductor layer; wherein the third semiconductor layer is disposed between the first semiconductor layer and the semiconductor substrate; the third semiconductor layer is peripheral such that, when viewed from above, it surrounds the first semiconductor layer, and an end portion of the third semiconductor layer overlaps with an end portion of the first semiconductor layer, or wherein the third semiconductor layer surrounds the first semiconductor layer with no overlap between the third semiconductor layer and the first semiconductor layer; and the third semiconductor layer is not electrically connected to the first electrode.