To provide a planetary gear mechanism in which a decrease in rotation efficiency is suppressed.

The planetary gear mechanism includes: a first sun gear; a plurality of first planetary gears to be meshed with the first sun gear; and a first carrier to which the plurality of first planetary gears are rotatably coupled. The first sun gear has a first engaging part on the central axis of rotation of the first sun gear. The first carrier has a first part to be engaged, which is disposed on the central axis of rotation of the first carrier, and is engaged with the first engaging part in a relatively rotatable manner.