Abstract: A boost converter having a plurality of boost converter units in parallel operation. Within each boost converter unit, current transformers 22,23 are connected in series respectively to switching element 7 and diode 8. The current transformers 22,23 permit detected current therefrom to output current detection signals from current detection circuits. Then, the current detection signals within one converter unit 3 will be compared with those within the other converter unit 3A in parallel operation by comparing circuit 33. On the basis of a result of such comparison, D.C. output voltage Vout can be controlled to be equal to each other in each converter unit 3,3A.