A molecular pump has a casing having a gas inlet port and a gas discharge port, a stator disposed within the casing, a shaft disposed in the casing concentrically with the stator, a bearing rotatably supporting the shaft for undergoing rotation relative to the stator in a preselected direction of rotation, a rotor mounted on the shaft for rotation therewith in the preselected direction of rotation, and a motor for rotationally driving the shaft. A flange is connected to the casing at the gas inlet port. The flange has at least one mounting hole for receiving a fastener, a first elongated through-hole disposed in communication with the mounting hole, and a second elongated through-hole spaced apart from the first elongated through-hole in a direction toward a direction of rotation opposite to the preselected direction of rotation to define between the first and second elongated through-holes a deformable thin-wall portion for undergoing deformation due to a shock resulting from a torque acting on the casing in the preselected direction of rotation.