To improve the structure of a plug attaching/detaching type switch that can be applied even to a portable electronic unit by preparing a plug serving as a partial external appearance of the electronic unit main body and a plug tip key part which opens and closes an internal plug type switch that generates alarms when the plug is attached and detached as a knob.

When a plug 2 is inserted into a corner part of a casing 1 and fixed there, a casing external appearance corner part is formed by the plug 2. At the same time, a tip key part 2A of the plug 2 is put into the casing 1 and then inserted between the upper and lower electrodes of a plug type switch 7 to push up the upper electrode in a prescribed direction and accordingly to separated both electrodes from each other. In this case, the conduction is opened between the upper and lower electrodes since the part 2A consists of an insulating material. Under such conditions, no alarm is generated. When the plug 2 is picked and the part 2A is pulled out between the upper and lower electrodes of the switch 7, an alarm circuit closes by the contact condition secured between both electrodes and the alarms are produced. In such a constitution, the malfunctions of alarms can be prevented.