Gears are microfabricated down to diameters of a few micrometres. Natural macromolecular motors, of tens of nanometres in diameter, also show gear effects(1). At a smaller scale, the random rotation of a single-molecule rotor encaged in a molecular stator has been observed(2), demonstrating that a single molecule can be rotated with the tip of a scanning tunnelling microscope(3,4) (STM). A self-assembled rack-and-pinion molecular machine where the STM tip apex is the rotation axis of the pinion was also tested(5). Here, we present the mechanics of an intentionally constructed molecule-gear on a Au(111) surface, mounting and centring one hexa-t-butyl-pyrimidopentaphenylbenzene molecule on one atom axis. The combination of molecular design, molecular manipulation and surface atomic structure selection leads to the construction of a fundamental component of a planar single-molecule mechanical machine. The rotation of our molecule-gear is step-by-step and totally under control, demonstrating nine stable stations in both directions.