The resistance torques around the ankle joint differ between plantar flexion and dorsiflexion during passive movements at the same angle. Some conventional methods that have modeled the relationships between the joint angles and resistance torques using time-invariant elastic coefficients, which cannot account for the hysteresis property. Here, we propose a new modeling method using a time-varying elastic coefficient to represent the hysteresis of the ankle joint. In the present study, we used a custom-made motorized device to rotate the ankle joint passively and measured the joint angles and resistance torques from five healthy participants. By comparing the measured torques with the estimated values, we show that the proposed method can estimate the torque with less error than the conventional methods.