A floor hinge has a body case 6 with an opening 7 at a predetermined position of a piston rear chamber 5. The opening 7 is provided with a flexible diaphragm 8 which is fastened, by a plug 13, to the circumferential wall of the opening 7. The diaphragm is deformed in response to the change of the pressure of a working oil to prevent the breakage of the body case 6 especially during summer time when atmospheric temperature is remarkably changed. <IMAGE>
