A coupling device which is provided at a mouth 4a of a container accommodating a flexible container containing liquid and gas and which includes a plug 11 and a fitted member to be fitted in the plug 11, wherein the plug 11 includes a plug cylinder 11a, a liquid passage 16 communicating with the liquid and through which the liquid is let in and out, gas passages 17 communicating with the gas and through which the gas is let in and out, and fluid passages 19 communicating with a space between the container and the flexible container and through which fluid is let in and out.