The invention discloses a method for diagnosing degradation of a lubricant and degradation diagnosing apparatus of the lubricant. The degradation diagnosing apparatus (10) of the lubricant includes a soot amount estimator (100) which is configured to estimate an amount of soot formed in a combustion chamber (28) of an engine (2) in a predetermined period, accumulates the estimated amount of soot to acquire a cumulative value, and diagnoses the degradation of the lubricant based on the cumulative value. The soot amount estimator calculates the amount of soot based on a speed of the engine and a load parameter related to a load of the engine, and corrects, based on a temperature parameter correlated to the temperature of the combustion chamber, the calculated amount of soot into an increased value if the temperature parameter is lower than a predetermined first temperature. Accordingly, the diagnosing precision of the degradation of the lubricant can be improved by improving the estimation precision of the amount of the soot.