A planetary gear mechanism includes: a case; a first shaft having a connecting shaft; a second shaft having a receiving hole to rotatably receive the connecting shaft; a sun gear joined to the first shaft; a planetary carrier joined to the second shaft; planetary gears rotatably supported by the planetary carrier; and a ring gear provided on an inner surface of the case. The connecting shaft or the receiving hole is formed with a helical groove extending vertically and connected to a gap formed between an upper surface of the sun gear and a lower end surface of the second shaft. The second shaft is formed with a first lubricating oil passage connecting the helical groove to sliding contact parts of the second shaft and the case. When the first shaft is rotating, at least a part of each planetary gear is immersed in lubricating oil stored in the case.