To provide a lumbar support device of a vehicle seat, capable of efficiently improving a support force as a lumbar support.

A seat back frame 11 includes a waist support member 31 supported through a spring member 35. The waist support member 31 is configured so that dimensions of a vehicle width direction or a height direction becomes small by being pushed out forward. The spring member 35 is coupled with the seat back frame 11 by one end 35a and is coupled with the waist support member 31 by the other end 35b. The spring member 35 is elastically extended by the waist support member 31 being pushed out forward and the dimension of the vehicle width direction or the height direction becoming small, and is composed to increase the spring force to a rear side.