A vehicular electrical equipment unit heating and cooling system includes a vehicular electrical equipment unit 10 including a battery 21 and an inverter YBU5 22 which are accommodated in a primary air passageway 14, an air inlet 44 through which air in a passenger compartment 6 is admitted to the primary air passageway 14, an air outlet 46 through which air flowing in the primary air passageway 14 is discharged to the outside of the electrical equipment unit 10, a subsidiary air passageway 30 connected to and cut off from the primary air passageway 14 to make a closed circuit 60 when connected to the primary air passageway 14, and a fan 40 for generating a flow of air in the primary air passageway 14.