A carburetor comprising: a carburetor body having an air intake trunk having an air intake bore formed in an inside of the air intake trunk and a valve cylinder intersecting the air intake trunk and projecting from one side of the air intake trunk; and a piston-type throttle valve that is slidably fitted into the valve cylinder and opens and closes the air intake bore, in which a housing concave portion opening toward the air intake bore is formed in an end face of the throttle valve, a venturi block that is to be housed in the housing concave portion when the throttle valve is fully closed is fixedly attached to the air intake trunk, and a venturi that is made continuous to an upstream side and a downstream side of the air intake bore when the throttle valve is fully opened is formed in the venturi block, a connection terminal of an operation wire capable of opening the throttle valve through a pulling operation thereof is connected to the throttle valve in a position above the housing concave portion, the operation wire is arranged in parallel to a central axis of the throttle valve inside the valve cylinder, and a return spring that biases the throttle valve in a closing direction is housed inside the valve cylinder, characterized in that a small concave portion is provided in a ceiling face of the housing concave portion, and a connection hole into which the connection terminal is fitted is opened in a small ceiling face of the small concave portion, on an upper end face of the throttle valve an annular groove concentric thereto and passing outside the connecting hole is formed, and a hat-shaped regulation member having an annular flange in a lower end portion thereof is fitted to a cylindrical central land portion surrounded by this annular groove, wherein the flange is disposed so as to abut on a bottom face of the annular groove, and a lower end of the return spring is supported by this flange and the flange is sandwiched between the return spring and the throttle valve. a carburetor body having an air intake trunk having an air intake bore formed in an inside of the air intake trunk and a valve cylinder intersecting the air intake trunk and projecting from one side of the air intake trunk; and a piston-type throttle valve that is slidably fitted into the valve cylinder and opens and closes the air intake bore, in which a housing concave portion opening toward the air intake bore is formed in an end face of the throttle valve, a venturi block that is to be housed in the housing concave portion when the throttle valve is fully closed is fixedly attached to the air intake trunk, and a venturi that is made continuous to an upstream side and a downstream side of the air intake bore when the throttle valve is fully opened is formed in the venturi block, a connection terminal of an operation wire capable of opening the throttle valve through a pulling operation thereof is connected to the throttle valve in a position above the housing concave portion, the operation wire is arranged in parallel to a central axis of the throttle valve inside the valve cylinder, and a return spring that biases the throttle valve in a closing direction is housed inside the valve cylinder, characterized in that a small concave portion is provided in a ceiling face of the housing concave portion, and a connection hole into which the connection terminal is fitted is opened in a small ceiling face of the small concave portion, on an upper end face of the throttle valve an annular groove concentric thereto and passing outside the connecting hole is formed, and a hat-shaped regulation member having an annular flange in a lower end portion thereof is fitted to a cylindrical central land portion surrounded by this annular groove, wherein the flange is disposed so as to abut on a bottom face of the annular groove, and a lower end of the return spring is supported by this flange and the flange is sandwiched between the return spring and the throttle valve.