To incorporate more semiconductor elements in a small-sized lightweight power semiconductor module provided in an arc discharge device. [Means for Settlement] A power supply unit of an arc discharge device includes a semiconductor module 1, and a radiator fitted onto the semiconductor module 1. The semiconductor module 1 includes a module casing 2, and common units 3a to 3c retained by the modulecasing 2. Each of the common units 3a to 3c has: a ceramic substrate 50 having a circuit surface disposed with a semiconductor element 54, and a radiation surface on a side opposite to the circuit surface; and a package 35 that exposes the radiation surface, and seals the circuit surface with heat resistant resin. The radiator is fitted onto the module casing 2 to be thereby brought into abuttingcontact with all of the radiation surfaces of the common units 3a to 3c. Based on such a configuration, more semiconductor elements can be incorporated in a small-sized lightweight power semiconductor module.