PURPOSE: To provide a new caster for a chair, a stool or etc., which is smooth in slide guide between the rolling portion and a case and easy to assemble in such a manner that it is operated as a caster when not loaded and not operated as a caster when loaded because the rolling portion including the wheel retreats from a working position.

CONSTITUTION: A caster 1 for a chair has the rolling portion 3 provided with a wheel 33 and installed in a case 2 via a spring 5. The rolling portion 3 is vertically slid while being guided by a guide rib 21. A supporting frame 31 constituting the rolling portion 3 is furnished with the fork 36 and the top plate 37 of the wheel 33. The guide rib 21 is provided on the inside face of the case 2 and faced so as to sandwich the side face of the fork 36 at both sides. The fork 36 and the guide rib 21 have mutual face contact.