The secondary air injection system mass air flow sensor (1) is installed on the lower side of the air pump cleaner assembly (2) and measures the secondary air flow. The secondary air injection system mass air flow sensor controls the temperature of the hot wire to a certain amount. The heat generated by the hot wire is reduced as the secondary air flows around it. The greater air flow, the greater the heat loss. Therefore, the electric current supplied to hot wire is changed to maintain the temperature of the hot wire as secondary air flow increases. The ECM detects the secondary air flow by means of this current change.