A photodiode array (1, 20, 22) having a plurality of photodetecting channels which are formed in a semiconductor substrate of a first conductivity type having an incident surface side, into which, in use, light to be detected is made to enter, and an opposite surface side opposite to said incident surface side, the photodiode array comprising: accumulation layers of said first conductivity type and having an impurity concentration higher than in the semiconductor substrate, the accumulation layers being formed at said incident surface side, multipliers configured to avalanche-multiply carriers generated due to incidence of the light to be detected, said multipliers being formed on said opposite surface side, and each of said multipliers being formed of a first layer (62, 62A) at the opposite surface side, the first layer being of a second conductivity type opposite to said first conductivity type, and a second layer (61, 61A) of the first conductivity type formed in said substrate so that said first and said second layer form a PN junction; and resistors, each formed over said opposite surface side and electrically connected to the first layer (62, 62A) of a respective multiplier by a respective first wiring, said resistors being connected in parallel to each other, and said resistors being connected to a second wiring for connection to an amplifying circuit; said accumulation layers, said multipliers and said resistors being respectively formed for each of the respective photodetecting channels; wherein the photodetecting channels are separated from each other by separators formed around the respective multipliers; wherein the separators are formed by trenches, wherein a low-refractive-index material having a refractive index lower than that of the semiconductor substrate is formed in the trenches and wherein the trenches with the low-refractive-index material are formed so as to penetrate the semiconductor substrate from said incident surface side to said opposite surface side; and wherein the accumulation layers are electrically connected to each other by a transparent electrode layer formed on said incident surface side. accumulation layers of said first conductivity type and having an impurity concentration higher than in the semiconductor substrate, the accumulation layers being formed at said incident surface side, multipliers configured to avalanche-multiply carriers generated due to incidence of the light to be detected, said multipliers being formed on said opposite surface side, and each of said multipliers being formed of a first layer (62, 62A) at the opposite surface side, the first layer being of a second conductivity type opposite to said first conductivity type, and a second layer (61, 61A) of the first conductivity type formed in said substrate so that said first and said second layer form a PN junction; and resistors, each formed over said opposite surface side and electrically connected to the first layer (62, 62A) of a respective multiplier by a respective first wiring, said resistors being connected in parallel to each other, and said resistors being connected to a second wiring for connection to an amplifying circuit; said accumulation layers, said multipliers and said resistors being respectively formed for each of the respective photodetecting channels; wherein the photodetecting channels are separated from each other by separators formed around the respective multipliers; wherein the separators are formed by trenches, wherein a low-refractive-index material having a refractive index lower than that of the semiconductor substrate is formed in the trenches and wherein the trenches with the low-refractive-index material are formed so as to penetrate the semiconductor substrate from said incident surface side to said opposite surface side; and wherein the accumulation layers are electrically connected to each other by a transparent electrode layer formed on said incident surface side.