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 . <EOS>