A gate valve includes a valve body to be pressed against a peripheral surface around opening through which a processing target object is loaded and unloaded, pressed members arranged on a surface of the valve body around the opening, a main slider which slides in a direction parallel to the peripheral surface around the opening and pressing mechanisms, provided at the main slider, for pressing the respective pressed members. Each of the pressing mechanisms includes a cam having a protrusion for pressing the valve body against the peripheral surface around the opening and an inclined portion sloping downward from the protrusion. The pressing mechanisms serve to press the valve body in a direction substantially perpendicular to the peripheral surface around the opening in a state that the valve body is positioned to face the opening, so that the valve body is pressed against the peripheral surface around the opening.