1. A method for diagnosing mastitis of cows, comprising the steps of irradiating visual light rays and/or near infrared rays in a wavelength range of 400 to 2500 nm into urine, raw milk or a mammary gland of a cow, detecting an intensity of transmitted light rays, reflected light rays or transmitted and reflected light rays from said urine, raw milk or mammary gland, effecting multivariate regression, and diagnosing the presence of the mastitis of the cow.