To provide a variable inertial mechanism that can sufficiently suppress variation of torque of an internal combustion engine even when a rotation speed of the internal combustion engine is in a relatively low state.

The variable inertial mechanism includes: a drive shaft connection material 11, a relative rotation material 12, a support material 13, movable materials 14, 15 and pressing materials 16, 17. The driving shaft connection material 11 has a side surface 21b facing the relative rotation material 12 in a circumferential direction. The relative rotation material 12 has a side surface 31a facing the side surface 21b in the circumferential direction. The side surface 21b and the side surface 31a are formed so that the more separated from an axial line to a radius direction the gaps in the circumferential direction between the driving shaft connection material 11 and the relative rotation material 12 that are defined by the side surface 21b and the side surface 31a are, the larger the gaps are. The movable material 14 is configured to move along the side surfaces 21b and 31a in accordance with the relative rotation of the relative rotation material 12 when torque is applied to a crankshaft 2.