To reduce the number of types of backrest as a component by individually varying the heights of a backrest and the height of a back supporting point, whereby a chair can be used with the backrest of variable height and design.

This backrest structure of a chair includes a backrest 1 varied in height and design in its fitted state by changing four directions. The structure includes a fitting means 6 for fixing the backrest 1 in a mounting position 4 of a support member 3 in the locking state and rotating the backrest 1 in the released state. The backrest 1 is provided with the back supporting point in a position biased to the upper side or the lower side, and the part 2 of the back supporting point is fitted to the mounting position 4 of the support member 3.