To solve such a problem that the wiring work for the conventional explosion-proof indicator lamp from a control board to an indicator lamp housed in a case with an explosion-proof specification is carried out by using a thick steel conduit pipe because of large current, in conformity with the "factory electric installation explosion-proof guide", by disusing the thick steel conduit pipe, the case with an explosion-proof specification, and a reinforcing subsidiary work for mounting the case, and by simplifying the indicator lamp mounting work and recucing the construction cost.

A current is divided into small current conforming with the "explosion-proof guide" by a safety maintaining instrument. Therefore, it becomes unnecessary for the indicator lamp to be housed in the case with the explosion-proof specification while the current flowing into the cable becomes within a specification range of the "explosion-proof guide" and the wiring work using the conduit pipe becomes unnecessary.