A stay for opening and closing of a door does not become a hindrance when the door is open and is visually uncluttered. The stay has: a main body mounted to a housing; an arm having one end rotatably connected to the main body; a biasing element on the main body and biasing the arm so the arm rotates in at least one direction; an arm slider mounted to the arm and slidable in the longitudinal direction of the arm; and a mounting seat installed on the door and rotatably connected to the slider. As the slider is slidable relative to the arm and rotatable relative to the mounting seat installed on the door, an opening and closing force can be transmitted to the door from the one arm without requiring two links. Thus, the stay does not become a hindrance to a user and is visually uncluttered.