A load calculator for deriving an internal load at a target point in a target member in which three orthogonal axes (X, Y, Z) are defined with respect to the target member, the internal load occurring in response to an external load, comprising: - an external load estimator configured to estimate the external load acting on the target member based on an output from a load sensor which is provided near the target member, or on external conditions; characterised by - a coefficient storage unit configured to store a first coefficient and a second coefficient, wherein the first coefficient denotes an internal load acting on the target point when a unit load in the direction of one of the three orthogonal axes (X, Y, Z) acts on a set concentrated load point (S i) in the target member, and no other and no other external load and moment acts thereon, and wherein the second coefficient is an internal load acting on the target point when a unit moment around one of the orthogonal axes (X, Y, Z) acts on the set concentrated load point (S i) and no other external load and moment acts thereon; - a replacement deriving unit, wherein, assuming that the concentrated load point (S i) is not displaced, the replacement deriving unit is configured to derive three replaced loads (T1x, T1y, and T1z) in the directions of the three orthogonal axes (X, Y, Z) acting on the concentrated load point (S i) and three replaced moments (M1x, M1y, and M1z) around the three orthogonal axes (X, Y, Z) acting on the concentrated load point (S i) by replacing an external load acting on the target member; and - an internal-load deriving unit configured to derive an internal load acting on any target point as a sum of a product of the first coefficient and the replaced load and a product of the second coefficient and the replaced moment when the external load acts on the target member. - an external load estimator configured to estimate the external load acting on the target member based on an output from a load sensor which is provided near the target member, or on external conditions; - a coefficient storage unit configured to store a first coefficient and a second coefficient, wherein the first coefficient denotes an internal load acting on the target point when a unit load in the direction of one of the three orthogonal axes (X, Y, Z) acts on a set concentrated load point (S i) in the target member, and no other and no other external load and moment acts thereon, and wherein the second coefficient is an internal load acting on the target point when a unit moment around one of the orthogonal axes (X, Y, Z) acts on the set concentrated load point (S i) and no other external load and moment acts thereon; - a replacement deriving unit, wherein, assuming that the concentrated load point (S i) is not displaced, the replacement deriving unit is configured to derive three replaced loads (T1x, T1y, and T1z) in the directions of the three orthogonal axes (X, Y, Z) acting on the concentrated load point (S i) and three replaced moments (M1x, M1y, and M1z) around the three orthogonal axes (X, Y, Z) acting on the concentrated load point (S i) by replacing an external load acting on the target member; and - an internal-load deriving unit configured to derive an internal load acting on any target point as a sum of a product of the first coefficient and the replaced load and a product of the second coefficient and the replaced moment when the external load acts on the target member.