PROBLEM TO BE SOLVED: To radiate the hot air of a cylinder head to the outside, and also improve the permeability of inner sides of head covers, by forming an opening part opened toward the front, upper and rear parts of the cylinder head, between the right and left head covers with which the upper part of the cylinder head is covered. SOLUTION: First and second head covers 75, 76 are attached to a cylinder head 30 in which an intake/exhaust port is formed so that an engine 27 is made to look like large, and the upper part of a camshaft storage part 36 is covered with these head covers 75, 76. At this time, the head covers 75, 76 are arranged by being separated from each other, an opening part 98 opened to the front, upper and rear parts of the cylinder head 30, is formed, and a space among the cylinder head 30 and the head covers 75, 76 is opened to the atmosphere by this opening part 98. Also, an extended part 77 extended downward is formed in the bottom wall of the head cover 76, a plug lead-through opening is formed in the position of this extended part 77, where is opposite to an ignition plug 39, and cooling fins 79... are integrally formed on the peripheral surface of the extended part 77.