When valve speed increases by high rotation-ization of engine, 'Jamping' may occur by the factory valve springs. Moreover, since inherent pitch is low, 'Surging' may occur in a high rotation region. In JUN, the valve spring with which the load optimal at the time of a set and a valve lift is acquired was manufactured. Even when the amount of valve lifts increases by using a high cam, since the adhesion margin is secured enough, the JUN valve springs does not have degradation and its durability is also improving.