To avoid the frequent stop of a machine to clean a protective element, and to ensure the safety of a person working together with the machine.

A protective device for a dangerous zone of a machine comprises two parallel slide parts 10 arranged along two side parts of the dangerous zone, and two switches 11 connected in series to at least one power source of a motor of the machine. One of these switches is arranged adjacent to one end of one of the slide parts 10, and the other one 11a is arranged adjacent to the other end of one of the slide parts. This device is also a protective element 8 sliding in the slide parts, the protective element 8 operates control members of two switches 11, 11a at the operation position and has a protective element having the length to close them.