A speed reduce according to an embodiment of the present invention includes a case having an outer diameter D, a plurality of crankshafts, and a carrier supporting the crankshafts in a rotatable manner. The carrier is rotatable by the crankshafts relative to the case. Each of the plurality of crankshafts includes a plurality of eccentric members each of which has a diameter dc1 and a crank journal that has a diameter dc2. The crankshaft further includes n1 needles that are arranged around each of the plurality of eccentric members and each of which has a diameter size dr1, and n2 needles that are arranged around the crank journal and each of which has a diameter size dr2. In the speed reducer, relational expression 5.5≤D/dc1≤7.0 is satisfied.