A transformer includes a main body and a secondary winding. The main body has a primary winding partition, several secondary winding partitions and two end portions. Several leads are disposed in either end portion. A wire-guiding area is formed in a sunken area in the primary winding partition and a buffer region is formed between the primary end portion and the primary winding partition. The secondary winding is wound around the core on the secondary winding partitions. One end of the wire runs from the secondary winding partitions to the wire-guiding area. Before the secondary wire goes into the buffer region, the secondary wire is folded for several times and then is twisted and rolled for several turns to make the secondary wire thicker. Then the secondary wire goes into the buffer region and then is connected to one of the primary leads. Due to the wire-guiding area and buffer region, the secondary wire would not be affected or pressed against by the primary wire.