To solve problems that as a driving means of an opening and closing valve for a secondary air passage, a means using negative pressure has concern for failure in highland having low atmospheric pressure, a means using discharge pressure of an air pump has inferior responsiveness and a means using an electric actuator causes voltage drop of an air pump (motor) and increase in a cost.

This secondary air supply system uses a means to drive the opening and closing valve 13 by using discharge pressure of the air pump 12, however, a structure is introduced that a diaphragm chamber 44 is arranged in the vicinity of an outlet port 54, and a part of discharge air is directly led to flow into the diaphragm chamber 44. Hereby, even in the highland having low atmospheric pressure, the opening and closing movement of the opening and closing valve 13 can be certainly performed and opening and closing responsiveness is excellent. An electromagnetic actuator is not used for driving the opening and closing valve 13, therefore, there is no failure that supply voltage of the air pump 12 (dc motor 29) drops and increase in the cost can be suppressed. Further, the number of lead wires 65 to be used can be reduced.