To suppress the overall length of a camshaft including the length of a decompression device provided in an engine and also to suppress an increase in number of parts of the decompression device, an engine includes a decompression device having a decompression weight pivotably supported through a pivot shaft to a camshaft and adapted to be rotated at a predetermined angle by a centrifugal force generated during the rotation of the camshaft. A weight accommodating portion for pivotably accommodating the decompression weight is formed between the opposite end portions of the camshaft. The outer diameter of the decompression device mounted to the camshaft is smaller than that of a ball bearing. The decompression weight is directly engaged with one end of a decompression camshaft to thereby rotate the decompression camshaft.