A chain guide structure for a chain drive type transmission mechanism which ensures that even when a solid matter, such as a small stone, gets nipped and a strong impact is exerted on a chain guide, the chain guide deforms so as to absorb the impact and permit the small stone or the like to be swiftly eliminated. The chain guide fastened to the engine case is provided so as to cover a drive chain wound around a drive sprocket, at a part of the periphery of the drive sprocket and in an arcuate form. The chain guide is formed with a brittle portion lower in strength than other portions thereof. The brittle portion is located above and in the vicinity of a front end portion at which the chain guide is situated closest to the drive chain. Thus, an engine case can be protected from damage.
