A pump includes a main body, a removable cover, an urging plate, an elastic member, and a control knob. Thus, the removable cover is locked on and unlocked from the main body easily, rapidly and conveniently by rotation of the control knob, thereby facilitating the operator mounting the removable cover. In addition, the locking portion of each of the two end portions of the urging plate is closely locked in the locking hook of a respective one of the catch members of the main body by rotation of the control knob, so that the removable cover is locked on the main body rigidly and stably.