There is provided a transformer capable of significantly reducing leakage inductance while satisfying safety standards. The transformer includes: a winding part having a plurality of coils wound on an outer peripheral surface of a cylindrically-shaped body part while being stacked thereon; and a terminal connection part extended from one end of the winding part in an outer diameter direction and having a plurality of external connection terminals coupled to a distal end thereof, wherein the terminal connection part includes at least one lead groove formed in a radial direction and at least one catching groove formed in the lead groove in a manner in which a width of the lead groove is extended in a winding direction of the coils.