A heatsink apparatus performs cooling by circulating a working fluid and causing a phase change between a liquid phase and a gas phase. The heatsink apparatus is provided on an external wall with a heat-generating body. The heatsink apparatus includes a box-shaped heat-receiving unit transferring heat to a heat-receiving plate; an inlet pipe supplying the working fluid to the heat-receiving unit; an outlet pipe discharging vapors into which the working fluid supplied to the heat-receiving plate is evaporated by heat; and a heat dissipater provided at a location higher than the heat-receiving unit and dissipating heat of the vapor passing through the outlet pipe. Slits are provided to the heat-receiving plate on a surface surrounded by an external circumference of the inlet pipe toward outside of the heat-receiving plate.