A diagnostic system for a power converter which includes a semiconductor device, and which performs a switching operation between conduction and interruption of the principal current flowing through a main circuit. The diagnostic system includes a current change amount calculation circuit for obtaining numeric data which reflects a current change amount of the principal current per unit time, a determination circuit for determining a state of the power converter by comparing the numeric data with a reference value, and an output circuit for outputting a determination result of the determination circuit.