To provide an automatic fastening type plug for fluid injection devised to be automatically tightened and fixed on an inner surface of a hole for fluid injection.

The automatic fastening type plug to use at the time of injecting fluid such as an adhesive, etc., at high pressure by forming the hole 36 for fluid injection on a structural body 34 on which a crack 34a, etc., are produced is constituted by including an inside member 3 having a central hole 3a and a lateral hole 3b, an outer peripheral surface of which is made a roughly conical surface 3d to be reversed taper and on an inner peripheral surface of which a female screw 3c to be srewed with a nozzle 7 is furnished and an outer sleeve 5 mounted on an outer periphery of the inside member 3 free to slide in the axial direction of the inside member 3, having a valve part 5a to close the lateral hole 3b free to open and close and a conical inner surface 5c to supplementally match with the reversed taper conical surface 3d of the inside member 3 and made of an elastic material smaller in frictional force between the inside member 3 and itself than frictional force between the hole 36 for fluid injection and itself.