1. A device for detecting a refrigerant quantity circulating in an air conditioner having a refrigerant circuit and a refrigerant receiver disposed upright across the refrigerating circuit for temporary storage of the refrigerant, the device comprising: a sensor which includes a body having an integral threaded base portion threadedly fitted in the refrigerant receiver through a bottom thereof, a hollow portion integral with said threaded base portion and extending from the threaded base portion inwardly of the receiver, said hollow portion having an open interior so as to permit the refrigerant in the refrigerant receiver to flow into said hollow portion to a level substantially even with the refrigerant level in the refrigerant receiver; a pair of electrodes disposed on an inner end wall of said hollow portion; and a floating element disposed within said hollow portion, said floating element being made of an electrically conductive material having a specific gravity a little smaller than that of the refrigerant; and an alarm means including an electric circuit electrically connected with said electrodes of said sensor, said alarm means comprising a differential amplifier having two input terminals and an output terminal; means for applying a fixed electric voltage to one of said input terminals of said differential amplifier; a circuit with a predetermined time constant coupled to said electrodes of said sensor and arranged to apply an electric voltage which is variable in response to the action of said sensor to the other input terminal of said differential amplifier; and indicating means substantially directly connected to said output terminal of said differential amplifier for actuation in response to an output from said differential amplifier to indicate when the refrigerant level in said refrigerant receiver falls below a given level.