When a capacity coefficient (Cre) of a torque converter is larger than or equal to a predetermined threshold (CreA), a speed ratio (e) is calculated on the basis of an actual power transmission efficiency (η) by referring to a predetermined unique relationship between a power transmission efficiency (η) and a speed ratio (e). Therefore, even in a second speed ratio variation range (R2) in which the capacity coefficient (Cre) is larger than or equal to the threshold (CreA) and the speed ratio (e) is not uniquely determined for the capacity coefficient (Cre), the speed ratio (e) is calculated using the unique relationship between the speed ratio (e) and the power transmission efficiency (η), so the speed ratio (e) may be calculated in all the speed ratio variation range of the torque converter.