A speed reducer capable of reducing vibration by tooth hitting noise while allowing, to some degree, the backlash of a pair of gears, wherein a pinion shaft is positioned parallel with a gear shaft, a pinion (1) formed on the pinion shaft and a gear (2) installed on the gear shaft are disposed to be meshed with each other to transmit the rotation of the pinion shaft to the gear shaft. The speed reducer comprises a housing (11) for storing the pinion (1) and the gear (2). The housing (11) comprises an internal surface of such a shape that the width thereof between the end part of an arc portion storing the gear (2) and the end part of an arc portion storing the pinion (1) in a direction vertical to the axial direction of the pinion shaft and the gear shaft in a cross section vertical to the axial directions of the pinion shaft and the gear shaft is monotonously increased from the pinion side to the gear side. The housing (11) contains an oil lubricant (14) of such an amount that can immerse therein a part of the meshed portion of the pinion (1) with the gear (2).
