Disclosed is a method of manufacturing a semiconductor device comprising forming an element isolation trench in a semiconductor substrate, coating a polysilazane perhydride solution on the semiconductor substrate having the element isolation trench formed thereon to form a polysilazane perhydride film, the polysilazane perhydride solution comprising dibutyl ether having a butanol concentration of 30 ppm or less, and polysilazane perhydride dissolved in the dibutyl ether, subjecting the polysilazane perhydride film to oxidation in an atmosphere containing water vapor to form a silicon dioxide film, and selectively removing the silicon dioxide film to leave the silicon dioxide film in the element isolation trench to form an element isolating insulation film.