A gear comprised of a tooth surface and a hard carbon film formed on at least a part of the tooth surface. When the gear is used in lubricant including a specific component, a friction of a tooth surface of the gear is largely decreased, and therefore the gear performs an excellent power transmission efficiency. Further, when the gear is employed in a planetary gear mechanism or speed reducing mechanism which has a plurality of meshing portions of gears, the power transmission efficiency of the mechanism is also improved.