A boiling refrigerant type cooling system to suppress overshoot upon start of heat generation and realize stable start of boiling. In the boiling refrigerant type cooling system, a metal boiling heat transfer unit has a base in thermal contact with a heat generating body. The boiling heat transfer unit is in contact with a liquid refrigerant. The boiling heat transfer unit has plural parallel tunnels communicating with the outside via holes or gaps under its surface, a groove deeper than a tunnel diameter formed through all the tunnels in an orthogonal direction to the tunnels, and a cover plate on the groove.