A four-cycle internal combustion engine with a valve system having improved durability in the case of high revolution speed. The engine includes a cylinder head having a port, and a valve having a stem part continuous with a fillet part for opening or closing the port. A valve spring including inner and outer springs urges the valve in a direction to close the port. A valve guide holds the stem part slidably. A tubular spring guide supports an outer periphery part of an outer spring to prevent lateral deformation of the spring. A top end of the spring guide is positioned above a top end of a valve guide.