An LD module cooling device includes, in a cooling plate, common flow paths that supply/drain a cooling medium in parallel to/from a plurality of cooling portion flow paths that correspond to a plurality of LD modules, in which the cooling portion flow path is a thin layer flow path having a flow path height and a flow path width that are constant in at least a majority of a flow path length, a rectangular shape of the cooling portion flow path defined by the dimensions flow path length×flow path width overlaps with at least the majority of a main contact surface between the cooling plate and the LD modules as viewed from a front surface of the cooling plate, the flow path height of the cooling portion flow path satisfies at least either one of a condition that flow path height is 1/20 or less of the flow path length and the flow path width, and a condition that the flow path height is 0.5 mm or less, and pressure loss of a cooling medium in the cooling portion flow path is greater than pressure loss of a cooling medium in the common flow paths.