The electronic circuit device is provided with: a power conversion circuit having a switching element incorporated therein; and a circuit substrate on which a circuit element constituting the power conversion circuit is mounted and a wiring pattern configured to electrically connect circuit elements is disposed. The power conversion circuit is composed of a pair of switching elements configured to supply energy to a choke coil intermittently in phase. The circuit substrate is constituted of a double-sided substrate in which one switching element is mounted on one main surface while the other switching element is mounted on the other main surface and a wiring pattern is provided in a manner such that a current path including one switching element and a current path including the other switching element overlap each other in plan view and have opposite current directions.