We developed high-speed differential-susceptibility measuring equipment to examine thermal fluctuations in magnetic materials. In this study, we measured the dependence of the differential-susceptibility of a magnetic tape on a magnetic field by using the equipment. When the sweep speed of the magnetic field changed from high to low in the order of ms, the magnetic field that gave the maximum differential susceptibility shifted from a high to a low magnetic field due to thermal fluctuations. However, when the sweep speed of the magnetic field changed from high to low in the order of μs, the shift in magnetic field was not clearly detected. The time when the magnetization in the easy direction changed from the residual to the zero state obtained from a linear field, and the time obtained from the constant field, which is the same as the coercivity of the linear field, were calculated using micromagnetic simulations. From the ratio of the two times and from an experimental time when the applied field reached coercivity, a time that could be applied to Sharrock's equation was obtained. In order to fit the experimental results to Sharrock's equation, K_u must be much smaller than the experimental value and H₀ must be much smaller than H_k (H₀/H_k=0.38). Here, K_u, H_k, and H₀ are anisotropy constat, anisotropy field, and coercivity at 0K.