We have constructed a planer undulator that is Halbach type composed of permanent magnet blocks. The period length of the undulator and the number of periods are 100 mm and 25, respectively. The vibrating wire method is studied to measure the periodic magnetic field of the undulator. A thin copper-beryllium wire is placed on the beam axis in the undulator. An AC current flow is applied in the wire, then by measuring amplitudes and phases of standing waves excited on the wire, we can reconstruct the magnetic field distribution along the wire numerically. In this paper, we discuss relations between reproducibility of the undulator field and the mode harmonics number incorporated into the reconstruction of undulator field.