A bearing diagnosing method for diagnosing degradation of a bearing rotatably supporting a spindle includes: the steps of obtaining the rotation speed of the spindle; obtaining the temperature of the bearing; recording the rotation time for which the spindle has rotated, at each of multiple classes that are defined in advance by combination of rotation speed range and temperature range, into the storage unit, based on the rotation speed and the temperature; and calculating bearing degradation information representing the degree of degradation of the bearing, based on the rotation time at each of the classes, stored in the storage unit.