An apparatus for measuring a cylindrically-shaped object, comprising: a measuring head including a measuring head support shaft; an inner surface measuring device being held by the measuring head and having three first distance sensors (3a, 3b, 3c) disposed in such a manner that the measuring directions of the first sensors are radial with respect to the center of the measuring head support shaft and are in the same plane with each other, the inner surface measuring device being configured to measure the distance to the hollow inner surface of the object; an outer surface measuring device being held by the measuring head and provided with a second distance sensor, configured to measure the distance from the second distance sensor to the outer surface of the object; the second distance sensor being disposed in such a manner that the measuring direction of the second distance sensor is in the same plane as that of the inner surface measuring device, opposed to the measuring direction of any one of the first distance sensors (3a, 3b, 3c) and being positioned at a predetermined distance outward from a region enclosed by the first distance sensors (3a, 3b, 3c); a displacement measuring device provided with a third distance sensor, configured to measure the displacement in a height direction of the measuring head; a position calculating means for calculating the position, with respect to the shaft center, of each of the measurement points on the hollow inner surface of the object that are measured by the at least three first distance sensors (3a, 3b, 3c), from the sum of the distance to the hollow inner surface of the object from the respective first distance sensor (3a, 3b, 3c) and the distance to the respective first distance sensor (3a, 3b, 3c) from the shaft center of the measuring head support shaft and calculating a point of equal distance from each measuring point as a center position of the object; and a dimensions calculating means for calculating dimensions of the object based on the results of output from the position calculating means and the results of measurement output from the outer surface measuring device and the displacement measuring device. a measuring head including a measuring head support shaft; an inner surface measuring device being held by the measuring head and having three first distance sensors (3a, 3b, 3c) disposed in such a manner that the measuring directions of the first sensors are radial with respect to the center of the measuring head support shaft and are in the same plane with each other, the inner surface measuring device being configured to measure the distance to the hollow inner surface of the object; an outer surface measuring device being held by the measuring head and provided with a second distance sensor, configured to measure the distance from the second distance sensor to the outer surface of the object; the second distance sensor being disposed in such a manner that the measuring direction of the second distance sensor is in the same plane as that of the inner surface measuring device, opposed to the measuring direction of any one of the first distance sensors (3a, 3b, 3c) and being positioned at a predetermined distance outward from a region enclosed by the first distance sensors (3a, 3b, 3c); a displacement measuring device provided with a third distance sensor, configured to measure the displacement in a height direction of the measuring head; a position calculating means for calculating the position, with respect to the shaft center, of each of the measurement points on the hollow inner surface of the object that are measured by the at least three first distance sensors (3a, 3b, 3c), from the sum of the distance to the hollow inner surface of the object from the respective first distance sensor (3a, 3b, 3c) and the distance to the respective first distance sensor (3a, 3b, 3c) from the shaft center of the measuring head support shaft and calculating a point of equal distance from each measuring point as a center position of the object; and a dimensions calculating means for calculating dimensions of the object based on the results of output from the position calculating means and the results of measurement output from the outer surface measuring device and the displacement measuring device.