An exchange chuck for exchangeably mounting a tool shank adapted for mounting a tool such as drill or tap. The tool shank is inserted to and fastened in a mounting bore formed in a tubular portion of a spindle. A movable sleeve is received on the tubular portion for movement longitudinally thereof, and plural balls are provided to the tubular portion so as to be intruded into or receded from said bore with movement of the movable sleeve, said balls operating to engage with and prevent extraction of the tool shank when intruded into the bore. An ejector member is received in the bottom region of the bore and biased forwardly at all times by a coil spring for extruding the tool shank forwardly at the time of exchange operation of the tool shank. The forward outer periphery of the ejector member is formed with an undercut annular groove contiguous to an engagement step operating to engage with the radially inner surfaces of the balls to prevent accidental extraction of the ejector member in the forward direction.