A small cooling device with multi-nozzle impinging jets is proposed for cooling an on-vehicle inverter with the heat generation of ~ 500 W/cm² . In order to minimize the cooling device, a double flow channel concept is introduced. In addition, to increase the critical heat flux, a heat transfer surface with uni-directional groove is utilized. 39 nozzles of 1.0 mm in diameter are set 2.0 mm above the heat transfer surface. The groove size is 1.0 mm x 1.0 mm. The experimental data shows that the critical heat flux of the cooling device exceeds 400 W/cm² and achieved 745 W/cm² at the maximum.