To provide a tilting device for a backrest of a reclining chair which is configured to steplessly stop a backrest at any tilt angle, has a simple structure, and easy to assemble in a short time.

A biasing unit A for biasing the backrest 9 to a rising direction includes: a pair of right and left turnably biasing devices B and B positioned at two parts of the right and left sides of the pivot shaft 8, and applying a turning force in the rising direction of the backrest 9 to a pivot shaft 8; and a gas spring 12 positioned between the right and left turnably biasing devices, having both ends connected to a support base 6 and the end of an arm portion 26 extending in a direction orthogonal to the pivot shaft 8, moving an operation knob 38 provided at one end to an operating position to be extended and contracted, and returning the operation knob 38 to a non-operating position so as not to be extended and contracted.