A method of manufacturing a trench gate semiconductor device having a diode formed by a pn junction between a first region of a first conductivity type and a second region of a second conductivity type, and at least two trenches with a mesa region in between. The method includes: forming the diode at a front side of the semiconductor substrate; forming an interlayer insulating film on the diode, the at least two trenches, and the mesa region; forming a plurality of contact holes penetrating the interlayer insulating film, and a plurality of contact trenches respectively continuous from the plurality of contact holes, thereby exposing a portion of each of the first region, the second region, and the mesa region, respectively; and ion-implanting an impurity of the first conductivity type in at least one of the exposed portions of the first region, the second region, and the mesa region.