To obtain an apparatus for measuring the load stress at a flange part of a steel structure having I-shaped cross section accurately in a short time using a stress measuring head having a magnetostrictive sensor.

The stress measuring apparatus comprises a stress measuring head 1 comprising a magnetostrictive sensor for determining the load stress of an object utilizing magnetic anisotropy generated through magnetostrictive effect where U-shaped yokes applied with exciting coil and detecting coil are arranged orthogonally in the center of the saddle part, a motor for turning the sensor and an encoder for measuring the turning angle of the sensor, a head 2 for demagnetizing the object, a supporting table 3 for mounting the stress measuring head 1 and the demagnetizing head 2, a mechanism for scanning the head supporting table 3 on the flange face thereof while being secured to a steel structure having I-shaped cross section, a controller for the scanning mechanism.