A passenger's weight measurement device for a vehicle seat, capable of suppressing generation of an initial load. The passenger's weight measurement device includes a pair of slide rails extending in parallel and fixed to the vehicle floor, and a pair of side frames that support the seat and are disposed above the pair of slide rails. Load cells are disposed between the pair of the slide rails and the pair of the side frames. A set of first connection frames for connecting both ends the pair of side frames extend in parallel so as to be perpendicular to the pair of side frames. Second and third connecting frames connect middle parts of the pair of slide rails so as to be perpendicular to the slide rails. The pair of the slide rails and the second and a third connecting frames constitute an H-shaped frame.