To actualize a simple structure and easy manufacture and assembling operation and to improve the easiness of use as an XY coordinate input device by accurately detecting the operation position of an operator.

The mouse device as a coordinate input device is equipped with a case 4 which is divided into two upper and lower parts so that they are coupled together detachably, the mouse ball 6 which is formed at the center part of the lower case 3 and stored detachably in a ball holder 5 having a lower-end opening 7, an annular cover 8 which is mounted detachably in the lower-end opening and holds the mouse ball, a couple of rotary bodies 9 and 10 which are stored in the case, have two mutually orthogonal axes 9A and 10A, and abut against the surface of the mouse ball, and rotation detectors 10 and 11 which detect the rotations of the rotary bodies and inputs XY coordinate signals to a computer part; and the mouse ball 6 is formed of a magnetized magnet ball 6b and rolls while attracted to a flat magnetic plate.