In the Tohoku University Heliac, transition to the improved mode has been observed when the plasma is biased with a hot cathode inserted into the plasma. Ion temperature measurement using high-resolution spectroscopy demonstrated that when the plasma was biased and transited to the improved mode, the ion temperature increased or was kept almost constant, and the ion density also increased. The total stored energy (the products of temperature and density, neTe, niTi) increased in the improved mode. In neoclassical theory, nonlinearity in ion viscosity plays a key role in the transition. The ion viscosity estimated from the measured ion temperature was consistent with the predictions of the neoclassical theory.