PURPOSE: To enable to detect directly existence of a residual material in a chuck, by a method wherein a pouring port of a fluid, which is attachable to and detachable from a rotor is provided on a stationary unit of the chuck and a piston pin protruding into the center of the chuck by a pouring fluid is provided on the rotor.

CONSTITUTION: The rotor consisting of a spindle 4, a hood 5 and a collet 6 is suspended so that a connecting hole 21 formed on the hood 5 faces an air pouring tube 11 connected with a cylinder 7 secured 8 to a stationary unit 3, in detecting existence of a bar material B or a bar material Bs whose length is less than a given one which are remained within a chuck 1. A pouring port 12 is sticked closely to a flange part 5a of the hood 5 by extending the cylinder 7 and compressed air is poured into an air chamber 19 through a flexible pipe 9, an air pouring tube 11 and a connecting hole 21. With this construction, existence of either a bar material B or a short bar material Bs is detected by making a piston 17 protrude into the center of the chuck by penetrating it through a long hole 25 of the collet 6.