A fluid-transfer coupling device includes a cylindrical housing (1), a cylindrical slider (25), which contains a valve sleeve (15) accommodating a valve (16) to be opened by pressure and which can slide in the cylindrical housing and plug supporting means (60), which supports a plug (70) on condition of being drawn up to an advanced end in the valve sleeve by the movement of the slider. With this constitution, even when using a coupling device having a relatively-large diameter, it is possible to supply fluid stably. Further the plug supporting means is arranged in front of the forward end of the slider. Additionally, the plug supporting means is provided with an improper connection preventing mechanism (68, 69a to 69d, 77) for preventing other plugs, except for a specified plug from being connected.