An LLC converter has high efficiency and low noise and has thus recently attracted attention in the field of power supplies for use in information and communication systems. A planar transformer is thought to be particularly effective in a high-frequency switching power supply because an ideal primary-secondary interleave structure can be formed by the multilayer structure, and the alternating-current (AC) resistance can be reduced. Based on these facts, we investigated the use of planar transformers in LLC converters. However, high-frequency oscillation, which is not observed in a normal winding transformer, appears in the secondary side current, and the power supply loss is also higher. Our investigation found that the current oscillation and an increase in loss were caused by a primary-secondary capacitance of the transformer. This paper presents countermeasures used to reduce the capacitance between the primary and secondary windings, and a new layer structure for the transformer that reduces the capacitance. The loss is calculated through a simulation and experimentally, and good agreement is obtained. The proposed transformer offers the high efficiency of 98.1% in a 200 W, 12 V output power supply.