A cooling device for electric equipment mounted in a vehicle, comprising: a heat source included in electric equipment, a cooling unit for cooling said heat source, said cooling unit including: a heat transfer member which has a first surface provided with said heat source and a second surface arranged on a back side of said first surface to form a heat transfer medium space arranged between said first surface and said second surface and transfers heat generated at said heat source; and a fin portion which is provided at said second surface and radiates heat transferred through said heat transfer member; characterized by a coolant passage forming member which is provided at said first surface and forms a coolant passage allowing a coolant for vehicle cabin air-conditioning to flow; and a supplying/discharging unit which supplies and discharges a heat transfer medium, which is different from the coolant flowing through said coolant passage, to and from said heat transfer medium space, and said heat source and said fin portion being arranged on opposite sides across said heat transfer member, and said coolant passage forming member and said fin portion being arranged on opposite sides across said heat transfer member. a heat source included in electric equipment, a cooling unit for cooling said heat source, said cooling unit including: a heat transfer member which has a first surface provided with said heat source and a second surface arranged on a back side of said first surface to form a heat transfer medium space arranged between said first surface and said second surface and transfers heat generated at said heat source; and a fin portion which is provided at said second surface and radiates heat transferred through said heat transfer member; characterized by a coolant passage forming member which is provided at said first surface and forms a coolant passage allowing a coolant for vehicle cabin air-conditioning to flow; and a supplying/discharging unit which supplies and discharges a heat transfer medium, which is different from the coolant flowing through said coolant passage, to and from said heat transfer medium space, and said heat source and said fin portion being arranged on opposite sides across said heat transfer member, and said coolant passage forming member and said fin portion being arranged on opposite sides across said heat transfer member. a heat transfer member which has a first surface provided with said heat source and a second surface arranged on a back side of said first surface to form a heat transfer medium space arranged between said first surface and said second surface and transfers heat generated at said heat source; and a fin portion which is provided at said second surface and radiates heat transferred through said heat transfer member; characterized by a coolant passage forming member which is provided at said first surface and forms a coolant passage allowing a coolant for vehicle cabin air-conditioning to flow; and a supplying/discharging unit which supplies and discharges a heat transfer medium, which is different from the coolant flowing through said coolant passage, to and from said heat transfer medium space, and said heat source and said fin portion being arranged on opposite sides across said heat transfer member, and said coolant passage forming member and said fin portion being arranged on opposite sides across said heat transfer member.