A slide door locking device for vehicles during feeding of fuel to a fuel tank positioned near a slide door, wherein a stopper lever is so pivotally mounted inside a panel of a body, the panel defining the rear end of a door opening, that the front end of the stopper lever may project into the door opening beyond the front end of the panel and engage with a stop member attached to the slide door, and an actuating lever is pivotally disposed to the panel, projecting into a hollow formed in the panel, for a fuel supply inlet, and is coupled to the rear end of the stopper lever via a link member. When the actuating lever is operated during feeding of fuel to a fuel tank, the front end of the stopper lever engages with the stop member of the slide door to prevent it from opening when it is opened. The stopper lever may be biased by a spring to a stop position so that the engage of the stopper lever with the stop member may be disengaged by further pivoting the stopper lever against the spring, thereby opening the slide door even when the fuel is fed to the fuel tank.