In a photodiode array 1, incident-side depressions 7 formed by thinning from the incident surface side of light to be detected are arranged in an array, whereas opposite-side depressions 11 formed by thinning regions corresponding to regions formed with the incident-side depressions 7 from the side opposite from the incident surface are arranged in an array. The bottom of the opposite-side depressions 11 is formed with a pn junction 3, whereby photodiodes of pn junction type are arranged in an array.