A method of manufacturing an optical sensor includes the steps of providing a semiconductor wafer having a plurality of pixel areas; forming a grid-like rib enclosing each pixel area on the semiconductor wafer, the grid-like rib having a predetermined width and being formed from a fixing member; providing a light-transmissive substrate having a gap portion on a main surface thereof, the gap portion having at least one of a groove having a width smaller than the grid-like rib and a plurality of through-holes; fixing the semiconductor wafer and the light-transmissive substrate such that the grid-like rib and the gap portion face each other; and cutting the fixed semiconductor wafer and light-transmissive substrate into pieces such that each piece includes one pixel area.