On the front side of an n-type semiconductor substrate 5, p-type regions 7 are two-dimensionally arranged in an array. A high-concentration n-type region 9 and a p-type region 11 are disposed between the p-type regions 7 adjacent each other. The high-concentration n-type region 9 is formed by diffusing an n-type impurity from the front side of the substrate 5 so as to surround the p-type region 7 as seen from the front side. The p-type region 11 is formed by diffusing a p-type impurity from the front side of the substrate 5 so as to surround the p-type region 7 and high-concentration n-type region 9 as seen from the front side. Formed on the front side of the n-type semiconductor substrate 5 are an electrode 15 electrically connected to the p-type region 7 and an electrode 19 electrically connected to the high-concentration n-type region 9 and the p-type region 11. This realizes a semiconductor photodetector and radiation detecting apparatus which can favorably suppress the occurrence of crosstalk, and restrain carriers from flowing into adjacent photodiodes even when a photodiode falls into an electrically floating state because of a breakage of a connecting point due to an initial connection error, a temperature cycle, etc.