Gears and coupling apparatus using the gears

Gears constituting a gear train and meshing with each other for enabling transmission of a power, wherein:

• the gears have tooth shapes formed such that a meshing line given by meshing of the gears is configured as a spiral mesh line, and the spiral meshing line extends from a point on a curve positioned on a radially inner side, which curve is one of two curves defining a meshing region, in a circumferential direction toward the side of broadening a distance between the two curves and intersects the curve positioned on the radially outer side.