Abstract: A third axis input device for a computer mouse includes a fixed primary seat, a fixed secondary seat, a wheel, an annular ring-like grating member, a suspending member, and an idle roller, characterized in that: the fixed primary seat has a lower portion provided with a first fixed rod and a first L-shaped hook member adapted to engage with a printed circuit board, a recess between the fixed rod and the L-shaped hook member for receiving a photoelectric controller, two arms provided at two sides of the fixed primary seat for engaging with the fixed secondary seat, a vertical rod and a vertical lug extending upwardly from the top of the fixed primary seat, the fixed secondary seat including a second fixed rod and a second L-shaped hook member having the same structure as the first fixed rod and first L-shaped hook member at the bottom rear portion, a slot at the bottom for receiving a photoelectric controller, and a hole above the slot, whereby the third axis input device is durable in use, simple in construct