To provide a defrosting controller for a freezer which is efficiently carries out defrosting operation at optimum start timing and finish timing.

The defrosting operation is started by opening a solenoid valve 28a of a bypass passage and communicating high temperature refrigerant from a compressor directly into a coil when a temperature difference between a coil outlet temperature of an evaporator from a coil outlet temperature sensor 51 and an internal temperature of a cold insulation box from a box internal temperature sensor 52 becomes ≥10°C. Refrigerating operation of lowering the temperature in the cold insulation box to an ultra low temperature is resumed by closing the solenoid valve of the bypass passage, finishing the defrosting operation and re-communicating refrigerant in the coil of the evaporator when the coil outlet temperature sensor reaches +7°C. Operation of a fan 24b for the evaporator is stopped from starting of the defrosting operation until the temperature difference between both temperature sensors becomes substantially equal by finishing the defrosting operation and resuming the refrigerating operation. Finishing of the defrosting operation within a predetermined time by a timer 53 is also carried out.