To improve the workability in assembling of a chain tensioner to an internal combustion engine.

The chain tensioner 210 for applying a tensile force to cam chains 37f, 37r transmitting the rotation of a crankshaft 2 includes a base frame 211 secured to front and rear banks Bf, Br, and is configured to be bent in an arched shape by biasing a slider plate 212 movable in the extending direction of the base frame 211 to elastically deform a tensioner 213 provided straddling between the base frame 211 and the slider plate 212. An upper securing portion 219 is provided to the upper part of the base frame 211 so as to be secured to the front and rear banks Bf, Br in the bending direction of the tensioner 213. The lower end of the base frame 211 is extended to a position facing the inside of the crankcase 3 to provide a lower securing portion 220 to be secured to the front and rear banks Bf, Br perpendicularly to the bending direction of the tensioner 213.