A port is formed in a cylinder head of an engine. The port is opened and closed by a valve. A valve spring device is provided with a first spring and a second spring. The second spring is contained in a housing and held by a stopper in a manner such that it is compressed along an axis or subjected to a preload. The preload of the second spring is larger than an installation load of the first spring. An axial load acts on the springs. The valve spring device has load-deflection characteristics such that only the first spring deforms axially when the axial load is not larger than a predetermined value and that both the first and second springs deform axially when the axial is load exceeds the predetermined value.