To provide a secondary air supply device that suppresses valve cut-off noise when an air switching valve is closed, at the end of operation of discharging retained water in a secondary air supply passage.

At the end of the operation of discharging the retained water which has retained in the secondary air supply passage 11 of the secondary air supply device 1, an air pump 12 is stopped and then after a predetermined time passes, the air switching valve 13 is closed. As a result, a flow rate of secondary air when the air switching valve 13 is closed is reduced or made "0", and the occurrence of the valve cut-off noise is prevented.