PURPOSE: To reduce interference noises and make assembling easy, by assembling a primary circuit and a secondary circuit of a switching power supply on a printed circuit board, and shielding only the primary circuit.

CONSTITUTION: Electronic circuit parts for a switching power supply are mounted on a printed circuit board 3. An output transformer is put at an almost central part of the printed circuit board 3, while a primary circuit and a secondary circuit are provided separately on different sides of the output transformer. A first upper shield metal member 10 has a U-shaped cross section to cover the circuit part in the primary circuit. A second lower shield member 11 fitted removably to the upper shield plate 10 is provided on a soldering face 3b. The second shield member 11 has a U-shaped cross section, forming a side part 20.