A spindle unit comprising a housing, a spindle being rotatably supported at a position radially inwards of the housing, a bearing supporting the spindle in a manner rotatable to the housing, and a rotational speed detecting means detecting a speed at which the spindle rotates relative to the housing, and a spindle condition detection device for a machine tool, said spindle condition detection device comprising: bearing load detecting means (16, 17) for detecting a load on the bearing; and present usage ratio calculating means for calculating, based on the load detected by the bearing load detecting means (16, 17), a present usage ratio that is a ratio of usage of the bearing with respect to a prescribed operating life of the bearing, which is achieved over a period until a present moment, characterized in that the present usage ratio calculating means divides a range of rotational speeds of the spindle into multiple rotational speed ranges, divides a range of loads on the bearing into multiple load ranges for each of the rotational speed ranges, and presets a prescribed operating life of the bearing for each of the load ranges for each of the rotational speed ranges; the present usage ratio calculating means calculates a divisional usage ratio of the bearing by dividing the operating time by the prescribed operating life, with regard to each of the load ranges for each of the rotational speed ranges; and the present usage ratio calculating means calculates the present usage ratio by summing up all the divisional usage ratios, wherein a load calculation portion calculates a load on the first bearing based on a radial displacement of the spindle with respect to a housing, the radial displacement being detected by the bearing load detecting means in the form of a radial displacement sensor, and based on an axial displacement of the spindle with respect to the housing, the axial displacement being detected by the bearing load detecting means in the form of an axial displacement sensor, wherein the present usage ratio calculating means calculates operating time in each of the load ranges for each of the rotational speed ranges, based on the load which is calculated by the load calculation portion and the rotational speed detected by the rotational speed detecting means. bearing load detecting means (16, 17) for detecting a load on the bearing; and present usage ratio calculating means for calculating, based on the load detected by the bearing load detecting means (16, 17), a present usage ratio that is a ratio of usage of the bearing with respect to a prescribed operating life of the bearing, which is achieved over a period until a present moment, characterized in that the present usage ratio calculating means divides a range of rotational speeds of the spindle into multiple rotational speed ranges, divides a range of loads on the bearing into multiple load ranges for each of the rotational speed ranges, and presets a prescribed operating life of the bearing for each of the load ranges for each of the rotational speed ranges; the present usage ratio calculating means calculates a divisional usage ratio of the bearing by dividing the operating time by the prescribed operating life, with regard to each of the load ranges for each of the rotational speed ranges; and the present usage ratio calculating means calculates the present usage ratio by summing up all the divisional usage ratios, wherein a load calculation portion calculates a load on the first bearing based on a radial displacement of the spindle with respect to a housing, the radial displacement being detected by the bearing load detecting means in the form of a radial displacement sensor, and based on an axial displacement of the spindle with respect to the housing, the axial displacement being detected by the bearing load detecting means in the form of an axial displacement sensor, wherein the present usage ratio calculating means calculates operating time in each of the load ranges for each of the rotational speed ranges, based on the load which is calculated by the load calculation portion and the rotational speed detected by the rotational speed detecting means.