PURPOSE: To make stable lubrication performable all the time even if oil viscosity varies, by installing plural pieces of oil grooves, being different in a sectional form each, in a shaft or a main bearing.

CONSTITUTION: Plural pieces of oil grooves 34a and 34b are installed on a lubricating surface of a shaft or a main bearing 35 in a parallel manner. Each of these oil grooves 34a and 34b is being formed so as to be different in width and depth each. That is to say, the width and depth of the oil groove 34a should be set to an optimum value to oil in low viscosity. And, the width and depth of the oil groove 34b should be set a larger value than that of the optimum groove 34a to oil in high viscosity. Therefore, when oil viscosity is high, lubricating oil is mainly fed to the groove 34b but when the oil viscosity is low, the groove 34a becomes easy to be lubricated. Thus, even if there is a variation in the oil viscosity, each groove acts on each other so that stable lubrication is made performable all the time.