A small-locus machining apparatus is attached to the distal end of a wrist of a robot but operable independently of a robot body. First and second drives of the apparatus are mounted on a base in such a manner that respective output spindles are parallel with each other. First and second rotational arms are coupled to the output spindles through decelerators, which are directly connected to the drives, allowing rotation in a plane orthogonal to the output spindles. These rotational arms and links interconnecting the rotational arms constitute a link mechanism having, for example, five joints. A tool is attached to this link mechanism, and the rotational amounts of the first and second arms are detected to determine the coordinate position of the tool.