A refrigerating and air-conditioning apparatus comprising: a refrigeration cycle being constituted by connecting a compressor, a condenser, expansion means, and an evaporator, the refrigerant cycle performing a cooling operation; an evaporator heating device heating the evaporator; a drain pan receiving drain-water from the evaporator and draining the drain-water; a drain-pan heating device heating the drain pan; frost detecting means including a light-emitting element that emits light to the evaporator and a light-receiving element that receives reflected light from the evaporator and outputs a voltage according to the reflected light; and a control device controlling on-off operation of the evaporator heating device, characterized in that the control device preliminarily has a current defrosting cycle ranging from a start of a defrosting operation to a start of a next defrosting operation of a subsequent defrosting cycle, wherein when a defrosting start timing of the current defrosting cycle is reached, the control device determines an estimated frost layer thickness at the defrosting start timing of the subsequent defrosting cycle on the basis of the detection result obtained by the frost detecting means at the defrosting start timing of the current defrosting cycle, and determines whether or not defrosting operation at the current defrosting cycle is necessary on the basis of said estimated frost layer thickness, wherein if the control device determines that the defrosting operation is not necessary, the control device cancels the defrosting operation and continues the cooling operation, and wherein if the control device determines that the defrosting operation is necessary, the control device turns on the evaporator heating device and starts the defrosting operation. a refrigeration cycle being constituted by connecting a compressor, a condenser, expansion means, and an evaporator, the refrigerant cycle performing a cooling operation; an evaporator heating device heating the evaporator; a drain pan receiving drain-water from the evaporator and draining the drain-water; a drain-pan heating device heating the drain pan; frost detecting means including a light-emitting element that emits light to the evaporator and a light-receiving element that receives reflected light from the evaporator and outputs a voltage according to the reflected light; and a control device controlling on-off operation of the evaporator heating device, characterized in that the control device preliminarily has a current defrosting cycle ranging from a start of a defrosting operation to a start of a next defrosting operation of a subsequent defrosting cycle, wherein when a defrosting start timing of the current defrosting cycle is reached, the control device determines an estimated frost layer thickness at the defrosting start timing of the subsequent defrosting cycle on the basis of the detection result obtained by the frost detecting means at the defrosting start timing of the current defrosting cycle, and determines whether or not defrosting operation at the current defrosting cycle is necessary on the basis of said estimated frost layer thickness, wherein if the control device determines that the defrosting operation is not necessary, the control device cancels the defrosting operation and continues the cooling operation, and wherein if the control device determines that the defrosting operation is necessary, the control device turns on the evaporator heating device and starts the defrosting operation.