To realize stable input operation by practically extending the upper face of a mouse backward, to adjust the mouse in accordance with the size of a user's hand.

With respect to the mouse for operation of input to a personal computer or the like, an aid for mouse which extends the body of the mouse backward and is made of a bag body is fitted from the rear end of the mouse and is attached freely detachably to the mouse by a detaining means in the side part of the aid.