A transformer includes: a base that is a plate-like member and has a first surface and a second surface, a core attached to the first surface of the base, coils wound around the core, coil terminals attached to the second surface of the base, and a cooling unit that is thermally connected to the core and is to release heat transferred from the core. The coil terminals are disposed on the second surface of the base opposite to the first surface of the base to which the core is attached. The cooling unit is disposed on a side of the core opposite to the base.