An SOHC type engine in which installed in a cylinder head having an intake valve and a first ignition plug, and an exhaust valve and a second ignition plug respectively arranged side by side along the axis of a cam shaft are first and second insertion/removal guide sections for respectively guiding the first and second ignition plugs in insertion/removal, wherein a single rocker shaft (55) commonly supporting an intake−side rocker arm (59) driven by an intake−side cam (57) to drive an intake valve (47) and an exhaust−side rocker arm (60) driven by the exhaust−side cam (58) to drive an exhaust valve (48) is fixedly disposed above a cam shaft (34), and a first insertion/removal guide section (68) and the intake valve (47) are arranged to lie over each other at least partly on a projection plane orthogonal to the axis of the cam shaft (34), while a second insertion/removal guide section (69) and the exhaust valve (48) are arranged to lie over each other at least partly on the projection plane. This makes it possible to set the width of the cylinder head (22) in a direction orthogonal to the axis of the cam shaft (34) at a small value.
