To reduce the size and manufacturing cost of a surface mounting-type transformer.

In the surface-mounting type small-sized transformer provided with an upper core (1), a lower core (3), and a winding coil (2), the upper and lower cores (1) and (3) are respectively formed integrally in a cap-like shape composed of a central columnar section (4) and an outer peripheral section (7), which surrounds the columnar section (4) and has a rectangular external form and in a planar shape having the rectangular shape corresponding to the external form of the upper core (1).