Disclosed is a planar transformer including a first planar core which is formed of a magnetic substance, a lower secondary winding which is disposed to enclose a first left sill of the first planar core between a first rear sill and a first front sill of the first planar core; a primary winding which is disposed on the lower secondary winding so as to enclose first left and right sills of the first planar core; an upper secondary winding which is disposed on the primary winding to enclose the first left sill of the first planar core; and a second planar core disposed on the upper secondary winding.