To appropriately and inexpensively determine the quality of a lubrication state of a bearing whose lubrication state periodically varies.SOLUTION: In a lubrication state diagnostic apparatus 100, a main shaft 1 is rotated at a diagnosis rotational speed, and, at an inertia operation start time, the rotational speed is controlled to an inertia operation start speed and an inertia operation is started. During the inertia operation, the rotational speed is recorded in the time sequence, and, when the speed becomes an inertia operation end speed, the rotational speed is controlled to a diagnosis rotational speed. Then, an operation unit 9 calculates a feature quantity representing the magnitude of the rotational resistance of the main shaft 1, calculates a feature-quantity-calculation-time lubrication phase indicating the timing to which the representative time for calculating the feature quantity corresponds in the discharge period of a lubricant, and determines the lubrication state of a bearing 2a on the basis of a plurality of feature quantities having different feature-quantity-calculation-time lubrication phases.SELECTED DRAWING: Figure 1
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