A method for manufacturing a semiconductor device comprising: forming a drift region of a first conductivity type on a first main surface of a semiconductor substrate; forming a trench in part of an upper portion of the drift region; forming an electric field reducing region of a second conductivity type; forming an anode electrode embedded in the trench and connected on a top surface of the drift region; and forming a cathode electrode on a second main surface of the semiconductor substrate which is opposite to the first main surface, wherein an energy barrier is generated between the drift region and the anode electrode for blocking a movement of electrons from the anode electrode to the drift region when a positive voltage is applied to the cathode electrode with the anode electrode being reference, wherein the electric field reducing region is selectively formed in part of the upper portion of the drift region by ion implantation, the trench is formed by etching a portion of the drift region surrounded by the electric field reducing region and an inner portion of the electric field reducing region, the trench having a side surface surrounded by the electric field reducing region, and forming the trench includes: forming an etching mask having an opening portion through which a surface of the portion of the drift region surrounded by the electric field reducing region and a surface of the electric field reducing region are exposed; forming a side wall along the opening portion of the etching mask such that an end portion of the side wall is located above the electric field reducing region; and etching the portion of the drift region and the inner portion of the electric field reducing region by using the etching mask and the side wall as a mask, further comprising, after forming the trench: forming a sacrificial oxide film by oxidizing the electric field reducing region exposed on the side surface of the trench; and removing the sacrificial oxide film by etching to leave the electric field reducing region only in a corner portion of a bottom portion of the trench. forming a drift region of a first conductivity type on a first main surface of a semiconductor substrate; forming a trench in part of an upper portion of the drift region; forming an electric field reducing region of a second conductivity type; forming an anode electrode embedded in the trench and connected on a top surface of the drift region; and forming a cathode electrode on a second main surface of the semiconductor substrate which is opposite to the first main surface, wherein an energy barrier is generated between the drift region and the anode electrode for blocking a movement of electrons from the anode electrode to the drift region when a positive voltage is applied to the cathode electrode with the anode electrode being reference, wherein the electric field reducing region is selectively formed in part of the upper portion of the drift region by ion implantation, the trench is formed by etching a portion of the drift region surrounded by the electric field reducing region and an inner portion of the electric field reducing region, the trench having a side surface surrounded by the electric field reducing region, and forming the trench includes: forming an etching mask having an opening portion through which a surface of the portion of the drift region surrounded by the electric field reducing region and a surface of the electric field reducing region are exposed; forming a side wall along the opening portion of the etching mask such that an end portion of the side wall is located above the electric field reducing region; and etching the portion of the drift region and the inner portion of the electric field reducing region by using the etching mask and the side wall as a mask, further comprising, after forming the trench: forming a sacrificial oxide film by oxidizing the electric field reducing region exposed on the side surface of the trench; and removing the sacrificial oxide film by etching to leave the electric field reducing region only in a corner portion of a bottom portion of the trench. forming an etching mask having an opening portion through which a surface of the portion of the drift region surrounded by the electric field reducing region and a surface of the electric field reducing region are exposed; forming a side wall along the opening portion of the etching mask such that an end portion of the side wall is located above the electric field reducing region; and etching the portion of the drift region and the inner portion of the electric field reducing region by using the etching mask and the side wall as a mask, further comprising, after forming the trench: forming a sacrificial oxide film by oxidizing the electric field reducing region exposed on the side surface of the trench; and removing the sacrificial oxide film by etching to leave the electric field reducing region only in a corner portion of a bottom portion of the trench.