A clamp device comprising: a clamp arm extending along a housing in a horizontal direction, the clamp arm including a fulcrum portion which is swingably supported by the housing at a middle portion in the horizontal direction, an input portion which is provided at a base end thereof in the horizontal direction, and an output portion which is provided at a leading end thereof in the horizontal direction; a cylinder hole formed in the housing and below the input portion so as to extend in an up-down direction and be opened in an upper surface of the housing; a piston inserted into the cylinder hole, the piston being configured to advance to above an opening end of the housing to move the input portion upward; a working chamber which is formed between a bottom wall of the cylinder hole and the piston and which a pressure fluid is supplied into and discharged from; a first hole, for mounting a spring, formed near a leading end portion of the housing so as to be opened obliquely upward toward a leading end-adjacent portion of the clamp arm; characterised in: a through hole formed between the output portion and the fulcrum portion so as to extend obliquely upward and provide communication between an upper surface and a lower surface of the clamp arm, the through hole including a second hole for mounting a spring and a female screw hole which are formed so as to extend upward; a return spring mounted in the first hole and the second hole and configured to bias the output portion upward; and a pressing bolt screwed into the female screw hole from above to compress the return spring. a clamp arm extending along a housing in a horizontal direction, the clamp arm including a fulcrum portion which is swingably supported by the housing at a middle portion in the horizontal direction, an input portion which is provided at a base end thereof in the horizontal direction, and an output portion which is provided at a leading end thereof in the horizontal direction; a cylinder hole formed in the housing and below the input portion so as to extend in an up-down direction and be opened in an upper surface of the housing; a piston inserted into the cylinder hole, the piston being configured to advance to above an opening end of the housing to move the input portion upward; a working chamber which is formed between a bottom wall of the cylinder hole and the piston and which a pressure fluid is supplied into and discharged from; a first hole, for mounting a spring, formed near a leading end portion of the housing so as to be opened obliquely upward toward a leading end-adjacent portion of the clamp arm; characterised in: a through hole formed between the output portion and the fulcrum portion so as to extend obliquely upward and provide communication between an upper surface and a lower surface of the clamp arm, the through hole including a second hole for mounting a spring and a female screw hole which are formed so as to extend upward; a through hole formed between the output portion and the fulcrum portion so as to extend obliquely upward and provide communication between an upper surface and a lower surface of the clamp arm, the through hole including a second hole for mounting a spring and a female screw hole which are formed so as to extend upward; a return spring mounted in the first hole and the second hole and configured to bias the output portion upward; and a pressing bolt screwed into the female screw hole from above to compress the return spring.