We investigated the proton dynamics of crystalline bisquaric acid (H(2)BSQ) in terms of the temperature and frequency dependence of its permittivity. It was found that the dielectric response of the H(2)BSQ crystal exhibited two-step changes. With raising temperature, the dielectric permittivity increased slightly at 40 K. Whereas the temperature dependence of the permittivity at this temperature showed a large isotope effect, there was no frequency dependence of the permittivity. However, the permittivity increased largely at 290 K and showed distinct frequency dependence, although there was no-isotope effect on its characteristic temperature dependence. It was concluded that such a dielectric response of the H(2)BSQ crystal was derived from the migration of protonic soliton coupled with the polarity inversion along the quasi-one-dimensional hydrogen-bonded chain.