An aspect of the present disclosure provides a diagnosis device for an internal combustion engine. The internal combustion engine 1 includes a variable geometry type turbocharger 14, and the turbocharger includes a variable vane 28, a link mechanism configured to operate the variable vane, and an actuator 29 configured to drive the link mechanism. The diagnosis device includes a control unit 100 configured to control an opening degree of the variable vane by controlling the actuator. The control unit determines that an abnormality has occurred in the link mechanism of the turbocharger when an operating time of the internal combustion engine in a predetermined operating region exceeds a predetermined upper limit value and a differential pressure between a target boost pressure determined according to an operating state of the internal combustion engine and an actual boost pressure exceeds a predetermined upper limit value.