To obtain an open showcase having a function for monitoring frost state in which an abnormal frost state is detected when frost on an evaporator increases abnormally during cold insulating operation of the open showcase and an alarm is delivered to the outside.

An inner temperature sensor, an outer air temperature sensor, and a controller for processing the detection signals of respective sensors are provided in a showcase. When the variation rate of a temperature signal detected by the inner temperature sensor exceeds a preset threshold level, a decision is made that frost on an evaporator has increased abnormally and an alarm signal is delivered from the controller to the outside. Variation rate of the temperature signal detected by the inner temperature sensor is corrected depending on the variation rate of a temperature signal detected by the outer air temperature sensor and compared with a threshold value in order to decide abnormal frost.