To provide a seat device capable of automatically or manually adjusting the height and hardness or a shape of a seat surface in response to a state in traveling and stopping.

This seat device S for a motorcycle and a three-wheeled motorcycle, has a front seat 4 and a rear seat 5 formed by covering a cushion material 2 with a surface skin material 3, by arranging the cushion material 2 on a bottom plate 1. The seat device has an air cushion 11 and a side air cushion 21 arranged between the bottom plate 1 and the surface skin material 3, an air pump 80 for supplying gas, an air supply exhaust part 90 arranged between the air pump 80, the air cushion 11 and the side air cushion 21 and holding internal pressure in a predetermined value, and a control part 60 for controlling the air supply exhaust part 90 for changing the height and the hardness or the shape of the seat surface 4a on the basis of an external signal. The air cushion 11 is formed by mutually connecting the upper surface skin 14a and the lower surface skin 14b by a nonexpansible connecting thread 15.

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