Gear mechanism and robot

A gear mechanism, comprising:

• a first gear accommodated in a gearbox and rotatably supported around a first axis line;
• a second gear accommodated in the gearbox and meshed with the first gear, the second gear rotatably supported around a second axis line extending along a flat surface crossing the first axis line and having a smaller conical angle than the first gear;
• a holder which is attached to the gearbox, which is removable in a direction along the second axis line, and which supports the second gear in a rotatable manner with a bearing;
• and
• a second gear shim which is placed at a position between the holder and the gearbox and which can adjust a position of the second gear in the direction along the second axis line, wherein
• the second gear shim comprises:
• a through hole in which the holder is located in a state where the holder penetrates the second gear shim, and a path which extends from an outer circumference edge of the second gear shim to the through hole, wherein the path allows the holder to pass in a direction crossing the second axis line between an outside of the second gear shim and an inside of the through hole.