The invention provides a motor-operated valve including a planetary gear mechanism achieving a greater reduction gear ratio without increasing the number of teeth of the gears, thereby enabling downsizing. In a planetary gear mechanism 1, a sun gear 91 functioning as an input gear receives smaller load than an internally toothed output gear 45, so gears (second planet gears 95) having a smaller module are connected to the planet gears 93, and the sun gear 91 is in engagement with the second planet gears 95. The sun gear 91 is formed of a same module as the second planet gears 95. The rotation of the sun gear 91 is reduced in speed and transmitted to the second planet gears 95, and rotation of the second planet gears 95 rotated at a reduced speed is further reduced by the slight difference in the numbers of teeth of the ring gear 44 and the internally toothed output gear 45 before being output.