A load calculator includes a coefficient storage, a replacement deriver, and an internal-load deriver. The storage stores a first coefficient, an internal load acting on a target point in a target member when a unit load acts on a concentrated load point in the target member along one of three orthogonal axes, and as a second coefficient, an internal load acting on the target point when a unit moment acts on the concentrated load point around one of the axes. The replacement deriver derives a replacement load and moment by replacing an external load acting on the target member with six force components acting on the concentrated load point. The internal load deriver derives an internal load acting on any target point with the first coefficient, the replacement load, the second coefficient, and the replacement moment when the external load acts on the target member.