We constructed an undulator of Halbach planer type for generation of intense coherent terahertz radiation (THz) with the very short electron bunch. The length and the number of periods are 100 mm and 25, respectively. Its maximum magnetic field is 0.41 T and K-value is 3.82 with 54 mm gap. The vibrating wire method is studied to measure the magnetic field of the undulator. The numerical studies have been performed for the THz undulator and derived a relation between a reproducibility of undulator field and the number of the harmonic modes . A test experiment was carried out using 200 mm wire and a pair of permanent magnet blocks. The results of the test experiment are described.