To provide a detector for load on seat provided with structure allowing a dimensional error of a vehicular body or a seat affecting precision of load detection, and capable of detecting a load acting on the seat by simple constitution.

In this detector 10 with a load detecting means D provided between a rail 5 for supporting the seat 1 onto a floor face F and a bracket 6 for fixing the rail 5 onto the floor face F to detect the load acting on the seat 1, a connection member 9 fixed to the load detecting means D to input the load to the load detecting means D is pivoted to the rail 5 via a clearance allowing prescribed motion with respect to the rail 5, the load detecting means D is provided with paired fulcrums on the bracket 6, and a power point is provided between the fulcrums to fix the load detecting means D.