To inhibit a lost portable telephone from being called from another telephone set and, in addition, from displaying registered private information.

When incoming calls are made by a set number of times from the same telephone set during a set period of time ('Yes' in a step 113), an information displaying function, such as the memory dial, etc., is stopped (step 118) and call out is inhibited (steps 120 or 121). For counting the number of incoming calls, the counted number is reset to '1' when incoming calls are made from other telephone sets even when the number of incoming calls is only one during the counting operation in the case where the call-in system is set to 'Continuous'. When the call-in system is set to 'Normal', incoming calls made from other telephone sets are ignored during the counting operation.