A method of manufacturing a semiconductor integrated circuit device having on the same substrate both a high breakdown voltage MISFET and a low breakdown voltage MISFET is provided. An element isolation trench is formed in advance so that the width thereof is larger than the sum of the thickness of a polycrystalline silicon film serving as a gate electrode of a low breakdown voltage, the thickness of a gate insulating film and an alignment allowance in processing of a gate electrode in a direction orthogonal to the extending direction of the gate electrode and is larger than the thickness of the polycrystalline silicon film in a planar region not overlapping the gate electrode. It is possible to decrease the number of manufacturing steps for the semiconductor integrated circuit device.