The present invention provides a fluid coupler device and an interface structure of a daughterboard. A receptacle has a backward-movable floating mechanism and includes a receptacle-side slider, a first lock mechanism (first ball) functioning to couple and fix the receptacle to a plug, and a second lock mechanism (second ball, indentation) that prevents the backward movement. The plug includes a plug-side slider and an indentation. When the plug is inserted into the receptacle, the plug-side slider is prevented from moving forward by the first ball and only the housing of the plug moves forward. When connection between male and female members is complete, the first ball fits into the indentation and the plug-side slider moves forward to lock the first ball. The receptacle-side slider is pushed backward by the plug-side slider, the second lock mechanism is unlocked and the housing of the receptacle is moved backward by the floating mechanism. The fluid coupler device is capable of achieving parallel arrangement and centralized connection of light connectors or the like which are not subjected to large push-in forces.