Provide is a cooler in which the number of components can be decreased and the space can be reduced. The cooler includes: a cooling plate (10) having a flat shape and including a plurality of refrigerant passages (14) partitioned by a plurality of partition walls (13, 13A) parallel to each other along a longitudinal direction; a first cutout portion (11), which is formed at one end of the cooling plate in the longitudinal direction, and is opened on an upper side and at an end portion in the longitudinal direction while leaving both side walls (10c, 10d) in a width direction and one (13A) of the plurality of partition walls; a second cutout portion (12), which is formed at another end of the cooling plate in the longitudinal direction, and is opened on the upper side and at an end portion in the longitudinal direction while leaving both the side walls in the width direction; a first upper lid member (20), which is joined to the first cutout portion, and has a refrigerant inlet port (25) and a refrigerant outlet port (27) respectively communicating to a refrigerant inlet side and a refrigerant outlet side partitioned by the one of the plurality of partition walls; and a second upper lid member (30), which is joined to the second cutout portion, and forms, between the second upper lid member and the second cutout portion, a flow passage space (35) causing the refrigerant inlet side and the refrigerant outlet side to communicate to each other.