Because deformations are important indicators of the degree of stability during construction of rock structures, monitoring of deformation is a key element of observational method in tunnels. Monitoring can also help determine the depth of the loosened zone, movements on discontinuities, in-situ modulus, and other items that may be dealt with in design. We have studied the applicability of digital photogrammetry for monitoring of surface displacements in civil engineering. Digital Photogrammetry is the technique obtaining three dimensional coordinates of an object by means of digital photography. This photogrammetry has the advantage of measuring deformation of an object by some photos with easy measurements and excellent cost performance. Because the linearized equation to analyze the coordinates of object points has some rank defects due to no control points in tunnel, we present constrained least square estimation to measure the displacements without the incorporation of control point configuration. The results of experiments demonstrated that this measurement system could yield high accurate three dimensional coordinates of target points situated on an object. It is also shown that this digital photogrammetry can monitor the displacements of the tunnel accurately along with a capability of real-time measurement because of removal of human skills in measurement tasks.