A secondary air supply device for an internal combustion engine, the secondary air supply device includes a secondary air supply passage, an air pump, a control valve, a pressure sensor and an electronic control unit. The electronic control unit is configured to: (a) when the control valve is opened so that the secondary air supply passage is opened and the air pump is driven, control the air pump to maintain driving of the air pump and control the control valve to close so as to close the secondary air supply passage; (b) calculate a differential pressure between a maximum pressure and a convergence pressure of the pressures within the secondary air supply passage since just after the secondary air supply passage is closed; and (c) calculate a flow amount of secondary air supplied from the secondary air supply passage supplied to the exhaust gas passage based on the differential pressure.