An element isolation film is formed by filling an oxide in a trench formed in an element isolation region of a semiconductor substrate to thereby form an insulation film for element isolation. A method of forming the element isolation film includes a first step of depositing a material in a plasma state including oxygen and silicon on an inner surface of the trench while applying no bias voltage (or a relatively low voltage), and a second step of filling the material in a plasma state including oxygen and silicon in the trench while applying a bias voltage (or a relatively high voltage).