A strainer includes a strainer body, a slider to which a cam follower is fixed, a holder to which snare wires are fixed, an operating lever, and a cam that is pivoted through operation of the operating lever. The slider is slid relative to the strainer body by pivoting the cam through operation of the operating lever so that the cam follower moves along the cam surface. The movement distance of the slider linearly changes with respect to an operation angle of the operating lever.