A method for manufacturing a semiconductor device includes: digging first and second trenches at the top surface of a plate-like base-body portion; forming an insulating film in the inside of each of the first and second trenches; laminating a conductive film on the top surface of the base-body portion so as to bury the first and second trenches with the conductive film via the insulating film; testing insulation-characteristics of the insulating film by applying a voltage between the conductive film and the bottom surface of the base-body portion; and after testing the insulation-characteristics, selectively removing the conductive film from the top surface of the base-body portion, so as to define a gate electrode in the first trench and an separated-electrode in the second trench, the separated-electrode being separated from the gate electrode.