To provide a variable valve train shim rotation prevention structure for an internal combustion engine of easy shim mounting work and a simple structure, capable of preventing noise generation.

Butting surfaces 62c, 62d of the shim 62 are formed on an edge facing outside at both side of a support shaft storage space 62d, and are formed to make interval get narrower at an open side of a support shaft storage space 62b. When a rocker shaft 56 is inserted between an intervening drive mechanism 22 and a reference surface having two projections 70 from an upper part to be stored in a support shaft storage space 62b, side surfaces of the projections 70 naturally touch the shim 62 from both sides to grip the same, and insertion of the shim 62 is stopped in a state that the projections 70 touch the butting surfaces 62c, 62d respectively. An insertion position and a rotation position of the shim 62 are fixed by stop due to the touch. Even if drive force from an intake cam 28 is transmitted to the intervening drive mechanism 22, the shim 62 is not rotated together and a posture thereof is sufficiently stably retained. Consequently, the problem can be solved.