The allowable maximum tooth load of marine reduction gears has been described by various researchers, investigating the contact compressive stress of tooth surface and bending: stress of tooth root. In those researches, however, the load distribution along a pinion axis is generally assumed to be uniform. The load distribution along a pinion axis has been investigated by Poritsky and Lewis. But the former's analytical solution is very complicated, while the latter's cut-and-try methcd requires a great deal of labour. In this paper, the author assumed the load distribution as a sine function including two arbitrary constants, which are so varied as to satisfy the condition of pinion deformations, i. e., the summation of pinion bending deflection, pinion torsional deformation and tooth bending deflection is always constant along a pinion axis. This approximate solution, being sufficiently accurate for engineering purpose and very simple, briefly concludes that the most adequate length of pinion is twice of pitch circle diameter of pinion in ordinary tooth stiffness. Another fundamental problem is the tooth lubrication. This problem has been treated by K. Yokoyama and E. K. Gatcombe, but those investigations are only limited to the particular case of boundary conditions of oil film between tooth surfaces. In this paper, the author introduced a solution under more general boundary conditions, assuming that the oil film is in a stable condition when the frictional coefficient between tooth surfaces is minimum at a certain oil temperature. According to the author's opinion, the oil film should always exist between tooth surfaces and torque should be transmitted through this film. So the tooth load is to be limited in the range in which the oil film is in stable condition and expressed in following equation. P={6.74-3.48(2-h_a1+h_a2/M_n)}3√2> where : P allowable maximum tooth load per pinion face width in kg/cm. h_a1, h_a2 pinion and wheel addendum. M_n module of gear. U peripheral velocity at pitch circle diameter in m/s. d pitch circle diameter of pinion in cm. R reduction ratio of gear.