A chain tensioner includes a base frame secured to each of front and rear banks, a slider plate shiftable in the extending direction of the base frame, and a tensioner provided to straddle between the base frame and the slider plate. The slider plate is biased to elastically deform and arcuately bend the tensioner, which applies a tensile force to each of cam chains transmitting the rotation of a crankshaft. In the chain tensioner, upper securing portions are provided in an upper portion of the base frame so as to be secured to each of the front and rear banks in the bending direction of the tensioner. A lower end of the base frame extends to a position facing the inside of the crankcase to provide a lower securing portion which is secured to each of the front and rear banks perpendicularly to the bending direction of the tensioner.