A cooling device is provided with an inverter compressor. Set speeds of the inverter compressor can be switched to six stages from a first speed to a sixth speed. The relationship between each of the set speeds of the rotational speed is set so that the stages adjacent to each other have gradually larger differences in rotational speed as the rotational speed becomes higher. With this, independently from the level of the rotational speed, a degree of increase in cooling performance between each of the stages can be equalized. In a case of performing a control to increase or decrease the rotational speed of the inverter compressor stage by stage depending whether an actual temperature drop rate is larger or smaller than a target temperature drop rate, the change amounts in cooling performance that can be substantially equalized, and too high or too low cooling performance does not result, i.e. fluctuation in cooling performance can be minimized. Thus, control to drop the internal temperature in accordance with a predetermined cooling characteristic can be stably performed.