To deem a dedicated oil passage unnecessary and therefore make an engine equipped with a plurality of cylinder bores with substantially horizontal axes and provided with transmission mechanisms between a camshaft and crankshaft within a valve chamber more lightweight and compact while returning oil for a valve chamber to a crank chamber. Transmission mechanisms include endless chains wrapped around drive sprockets fixed to one end of a crankshaft and driven sprockets fixed to one end of camshafts. The transmission mechanisms are housed in transmission chambers in such a manner that ends of the transmission mechanisms are made to communicate with the valve chambers and the other ends of the transmission mechanisms are made to face one end in the axial direction of the crankshaft. A return hole causing the lower part of the other end of the transmission chambers to communicate with the crank chamber is provided at the crankcases.