When a gas flow producer is driven, plural kinds of gases flow into a measure-passage at respective timing. An orifice is provided in the measure-passage. The orifice has a diameter-changing portion which restricts a separation of gases from an inner surface of the measure-passage. A pressure sensor is provided in the measure-passage to detect a pressure determined by the orifice and the gas flow producer. A microcomputer calculates a density ratio between the gases based on the detected differential pressure.