PURPOSE: To provide a defrosting device for a cooler in which draining can be conducted at an optimum draining time responsive to a defrosted amount of an evaporator.

CONSTITUTION: When it becomes a defrosting time, defrosting means 10 is operated by defrost control means 12 to heat an evaporator 5 of a cooler located in a cold gas circulation passage 6, a latent heat changing time at the time of defrosting is measured on the basis of temperature information from temperature detecting means 11 for detecting a temperature of the evaporator 5, and a draining time is obtained in response to the measured time and a length L of the evaporator 5 in a vertical direction. After the means 10 is stopped, draining is conducted for the draining time, and then a cooling operation is restarted.