A differential Bevel gear speed reducer consisting of standard type gears, each of the standard type gears having a same tooth form and a same gear module value, wherein three identical pinion Bevel gears are located between an input Bevel gear and an output Bevel gear, revolution-inducing Bevel gears that have a same number of teeth, a same module value, and a same cone apex as the pinion Bevel gears are fixedly placed at an outer side of axes of the three pinion Bevel gears, an orbit Bevel gear that is engaged with the revolution-inducing Bevel gears is fixedly placed at inside of a housing, causing the pinion Bevel gears and the revolution-inducing Bevel gear to revolve along an orbit of the orbit Bevel gear as the input Bevel gear revolves, and the rotation of the input Bevel gear causes differential rotation of the output Bevel gear.