The present invention provides a boosting transformer for microwave oven, which is low in high-frequency loss, hardly saturated, small in size and easy to produce. The boosting transformer includes a rod-like ferrite core; and primary, secondary and heater windings wound around the rod-like ferrite core and piled up side by side in a direction of an axis of the rod-like ferrite core. It also includes a metal core formed from a long metal thin plate rolled by a plurality of times into a square-ring having one inner diameter larger than each of outer sizes of the primary, secondary and heater windings and the other inner diameter larger than a total size of the three windings piled up side by side, the metal core being arranged opposite to the rod-like ferrite core through a gap so as to be fitted toward the rod-like ferrite core from the outside of the primary, secondary and heater windings.