An internal combustion engine 5 includes an intake valve spring 60 including a closely-wound section 62 and a sparsely-wound section 63. The closely-wound section 62 is provided so that elemental wire portions thereof are closely in contact with each other in the direction of the coil axial line L1 while an intake valve closes an intake port. The sparsely-wound section 63 is provided so that elemental wire portions thereof are spaced apart from each other in the direction of the coil axial line L1 while the intake valve closes the intake port. The coil outer diameter D62 of at least a part of the closely-wound section 62 is smaller than the coil outer diameter D63 of at least a part of the sparsely-wound section 63.