A drive transmission mechanism of a sheet transportation apparatus of the present invention comprises an upstream transportation roller gear and a planet gear for transmitting a first driving force from a driving source to the upstream transportation roller gear, and a release mechanism for preventing the upstream transportation roller gear and the planet gear from engaging with each other. The release mechanism prevents the engagement of the planet gear and the upstream transportation roller gear when the driven gear is driven to reversely rotate by a second driving force. The release mechanism prevents the engagement by moving a release member before the planet gear and the driven gear engage with each other. The release mechanism moves the release member by the driven gear being reversely rotated by the second driving force which is a driving force other than the driving source. This makes it possible to realize a drive transmission mechanism of a sheet transportation apparatus comprising a release mechanism capable of releasing the engagement of the driven gear and the drive gear for transmitting the driving force from the driving source to the driven gear, without the need of reversely rotating the driving source or the fear of impairing the driving force from the driving source.