A combination gear having a simple structure is provided at low cost without increasing a number of components. In at least two gears adjacent in an axial line direction and having a same rotation direction, three engaging portions arranged in a regular triangle are formed at an end of one of the adjacent gears and a regular triangle engaging hole is formed at an end of another gear. The engaging portions and the engaging hole fit together with play remaining, and when one of the gears rotates, the three engaging portions and three surfaces of the engaging hole engage such that the other gear rotates coaxially, and when rotation stops, the engaging portions and the engaging hole move in a direction orthogonal to the axial line by a play amount and become eccentric.