An optical sensor element includes: an n-type semiconductor region formed on a substrate; an i-type semiconductor region which is formed on the substrate between the p-type semiconductor region and the n-type semiconductor region and which is lower in impurity concentration than the p-type semiconductor region and the n-type semiconductor region; an anode electrode formed on the insulation film and connected to the p-type semiconductor region; and a cathode electrode formed on the insulation film and connected to the n-type semiconductor region. A reverse bias voltage Vb is applied when detecting the photocurrent, the reverse bias voltage Vb satisfying a following relation. V1