PURPOSE: To prevent an accident of human body due to misoperation of an operation switch during the inspection work by setting the operation switch of the other inspection port not to function when one inspection port is opened.

CONSTITUTION: When a person in charge opens a first inspection port and intends to perform inspection for example, opening of the first inspection port is detected by a first opening detecting means 101A. A first contact 101A1 is connected to a second operation switch 10B provided in proximity to a second inspection port, and it becomes off when the first opening detecting means 101A2 detects opening. Consequently, even though the other person in charge on a second entrance operates the second operation switch 10B to intend to operate by mistake, the operation signal of the second operation switch 10B is ineffective, and operation is not perormed. Hereby, an accident of human body by misoperation of an operation switch in the inspection work can be prevented.