A load is based calibrate on data collected under wide observation conditions and promptly calibrate loads of a plurality of wind turbine blades. A load measuring apparatus is applied to a wind turbine in which a pitch angle of a wind turbine blade is variable. The apparatus includes a sensor for obtaining a distortion of the wind turbine blade; a load calculating unit having a function expressing a relation between the distortion of the wind turbine blade and a load on the wind turbine blade, for obtaining the load on the wind turbine blade by applying to the function the distortion based on measurement data of the sensor; and a calibration unit for calibrating the function based on the measurement data of the sensor obtained in a pitch angle range and a rotational speed range of the wind turbine blade in which a variation between maximum and minimum aerodynamic torques is equal to or less than a predetermined value.