This cooling mechanism for a surface-mounted-type photoelectric conversion element is provided on a circuit board to which a surface-mounted-type photoelectric conversion element, which has a signal terminal that is connected to inner wiring and a terminal for fixation that is not connected to the inner wiring on a back surface thereof, is mounted, the cooling mechanism has a front-surface-side copper foil pattern to which the terminal for fixation is connected, a back-surface-side copper foil pattern, and a through-hole via which connects the copper foil patterns, a cooling member which is fixed to the circuit board so as to have contact with the back-surface-side copper foil pattern, and which cools the back-surface-side copper foil pattern.