A reduction gear capable of satisfying, at high level, each of: realizing a high reduction ratio; preventing backlash by securing rotation smoothness; and carrying out compactification and weight saving of entire mechanism is provided. The reduction gear of the invention is a modified crown reduction gear, comprising: a pressing mechanism 16 operated to rotate; a fixed crown gear 2 fixed to an external member 15; a movable crown gear 1, wherein the difference in the number of teeth between the gears is one; and an output axis 3 flexibly attached to the gear 1. The gear 1 engages with the gear 2 at a slant by pressing-force provided from the mechanism 16. In this instance, contact locations of teeth of the gears 1 and 2 are dispersed at two places existing at both sides between which a gradient center line intervenes.