tau-phase organic quasi-two-dimensional conductors have a 4-fold single Fermi surface (FS) from band calculation. At low temperature, the resistivity turns into semiconducting, whereas Shubnikov-de Haas oscillations have been observed. The amplitude of the oscillation is quite large because the system is near the quantum limit (N=2-state) under -27 T. The observed oscillations include two frequencies that are inconsistent with the band calculation. The reason of this discrepancy may be explained by the reconstruction of the FS. In this work, we examined very high magnetic field transport property up to 60 T and reconsider the low temperature crystal structure of this salt by diffuse X-ray study.