A planar transformer, comprising: a first planar core (1100,7100) including a first main body (1111,7111) which is formed of a magnetic substance, a first rear sill (1133,7133) which is formed at a left rear long side of the first main body, a second rear sill (1123,7123) which is formed at a right rear long side of the first main body, a first front sill (1131,7131) which is formed at a left front long side of the first main body, a second front sill (1121,7121) which is formed at a right front long side of the first main body, a first left sill (1132,7132) which is formed at a center portion between the first rear sill and the first front sill, and a first right sill (1122,7122) which is formed at a center portion between the second rear sill and the second front sill so as to be spaced apart from the first left sill; a lower secondary winding (1311,1312,7311,7312) which is disposed to enclose the first left sill of the first planar core between the first rear sill and the first front sill of the first planar core; a primary winding (1211,7211) which is disposed on the lower secondary winding so as to enclose the first left and right sills of the first planar core; an upper secondary winding (1313,1314,7313,7314) which is disposed on the primary winding to enclose the first left sill of the first planar core; and a second planar core (1400,7400) disposed on the upper secondary winding and comprising a second main body (1411,7411) which is formed of a magnetic substance, a third rear sill (1433,7433) which is formed at a left rear long side of the second main body, a fourth rear sill (1423,7423) which is formed at a right rear long side of the second main body, a third front sill (1431,7431) which is formed at a left front long side of the second main body, a fourth front sill (1421,7421) which is formed at a right front long side of the second main body, a second left sill (1432,7432) which is formed at a center portion between the third rear sill and the third front sill, and a second right sill (1422,7422) which is formed at a center portion between the fourth rear sill and the fourth front sill so as to be spaced apart from the second left sill. a first planar core (1100,7100) including a first main body (1111,7111) which is formed of a magnetic substance, a first rear sill (1133,7133) which is formed at a left rear long side of the first main body, a second rear sill (1123,7123) which is formed at a right rear long side of the first main body, a first front sill (1131,7131) which is formed at a left front long side of the first main body, a second front sill (1121,7121) which is formed at a right front long side of the first main body, a first left sill (1132,7132) which is formed at a center portion between the first rear sill and the first front sill, and a first right sill (1122,7122) which is formed at a center portion between the second rear sill and the second front sill so as to be spaced apart from the first left sill; a lower secondary winding (1311,1312,7311,7312) which is disposed to enclose the first left sill of the first planar core between the first rear sill and the first front sill of the first planar core; a primary winding (1211,7211) which is disposed on the lower secondary winding so as to enclose the first left and right sills of the first planar core; an upper secondary winding (1313,1314,7313,7314) which is disposed on the primary winding to enclose the first left sill of the first planar core; and a second planar core (1400,7400) disposed on the upper secondary winding and comprising a second main body (1411,7411) which is formed of a magnetic substance, a third rear sill (1433,7433) which is formed at a left rear long side of the second main body, a fourth rear sill (1423,7423) which is formed at a right rear long side of the second main body, a third front sill (1431,7431) which is formed at a left front long side of the second main body, a fourth front sill (1421,7421) which is formed at a right front long side of the second main body, a second left sill (1432,7432) which is formed at a center portion between the third rear sill and the third front sill, and a second right sill (1422,7422) which is formed at a center portion between the fourth rear sill and the fourth front sill so as to be spaced apart from the second left sill.