An anomaly determination device executes a first anomaly determination process and a second anomaly determination process. The anomaly determination device determines in the first anomaly determination process that an anomaly is present in a link mechanism when a fully-closed-time detection value is outside a closed-side normal range and when a fully-open-time detection value is outside an open-side normal range. The anomaly determination device determines in the second anomaly determination process that an anomaly is present in the link mechanism if the difference between the fully-closed-time detection value and the fully-open-time detection value is outside a normal distance range set in advance when it is not determined in the first anomaly determination process that an anomaly is present in the link mechanism.