To provide a chair with a tilting seat surface and a tilting unit for moderate training of muscular strength of the legs corresponding to walking while doing office work in a sitting state.

The chair with the tilting seat surface includes a leg part, a seat for a user to sit and the tilting unit. The tilting unit is removably arranged between the seat and the leg part for periodically inclining the seat. The seat is inclined so as to be lowered from a base position toward one edge in a first direction which is vertical to a perpendicular direction, at the same time the seat is inclined so as to be lowered toward a first edge in a second direction which is vertical to the perpendicular direction and orthogonal to the first direction. Thereafter the seat is moved back to the base position. The seat is inclined again so as to be lowered toward the one edge in the first direction and at the same time it is inclined so as to be lowered toward a second edge in the second direction which faces the first edge in the second direction. Thereafter the seat is moved back to the base position.