An internal combustion engine boost pressure diagnostic apparatus includes a boost pressure sensor, a throttle valve opening degree sensor, a bypass valve opening degree sensor and a control unit. The boost pressure sensor detects a pressure between a forced induction device and a throttle valve. The throttle valve opening degree sensor detects a throttle valve opening degree of the throttle valve. The bypass valve opening degree sensor detects a valve opening degree of a bypass valve. The control unit determines that the pressure between the forced induction device and the throttle valve is abnormal upon the boost pressure sensor detecting the pressure being equal to or larger than a prescribed value, the throttle valve opening degree sensor detecting the throttle valve opening degree being in a prescribed opening degree region, and the bypass valve opening degree sensor detecting the bypass valve opening degree being in a prescribed opening degree region.