To increase a heat radiating efficiency by heat radiating fins to prevent electronic parts in a case from being exposed in high-temperature atmosphere and, further, facilitate the displacing work of a cooling fan without deteriorating cooling performance in a case wherein the cooling fan is provided.

A main circuit substrate 33, on which various electronic parts and the like are mounted, is received in a case 21 while a heat radiating part 26 is provided at the lower part of front surface of the case 21. Heat radiating fins 29 are arranged at the heat radiating part 26 while the heat radiating part 26 is covered detachably by a cover 28. In this case, heat generating parts among the electronic parts or switching elements 35 are mounted above the main circuit substrate 33 so as to be contacted with the base part 31 of the heat radiating fins 29 and when the switching elements 35 generate heat, the heat is transferred to the heat radiating fins 29 and is dissipated to the outside of an instrument through the venting port 28a of the cover 28.