A planar transformer is disclosed. The planar transformer includes a first core, a second core, a third core, and a fourth core, which are sequentially disposed; a primary coil unit having multiple primary substrates through which the first to fourth cores penetrate and on which primary coil patterns are formed such that magnetic flux is generated in a first direction in the first and fourth cores and in a second direction in the second and third cores; and a secondary coil unit having multiple secondary substrates through which the first to fourth cores penetrate and on which secondary coil patterns are formed, the secondary coil patterns formed on a periphery of the first to fourth cores such that current induced by the magnetic flux flowing in the first to fourth cores flows therein, wherein the multiple primary and secondary substrates form a multi-layer structure.