An electrical receptacle unit includes at least one receptacle body (3) internally provided with a pair of slotted terminals (4) for insertion of a pair of blades (L) of a load-side electrical plug (P), a thermistor (7) encased in a protective tube (9) outwardly projectable from between the slotted terminals (4) of the receptacle body (3), a coil spring (13) for projecting the thermistor (7) out from the receptacle body (3), a relay (14) for enabling and disabling supply of electric power to the slotted terminals (4), a control circuit (19) which operates the relay (14) to cut off supply of electric power to the slotted terminals (4) when the temperature of the thermistor (7) reaches or exceeds a preset temperature, and a buzzer (23) operated by an output signal produced by the control circuit (19) when the temperature of the thermistor (7) reaches or exceeds the preset temperature. When the thermistor rises to or above the preset temperature owing to tracking or the like, supply of power to the load-side plug is cut off and the alarm is activated to produce a warning that the load-side plug has overheated.