PURPOSE: To prevent a catalyst from being stained and to obtain the subject catalyst for olefin polymn. by a method wherein a suspension of a Ziegler type catalyst solid in an inert medium is prepd. in a reaction space of a revolutional reactor in which reaction function and filtration function are mutually exchanged by revolution, and it is then filtered in a filtration space.

CONSTITUTION: A reaction space 7 of a polyfunctional revolutional reactor 3 having the reaction space 7 and a filtration space 6 and being arranged so as to exchange these spaces by revolution of 180° is positioned in the lower space and a suspension of a Ziegler type catalyst solid contg. a transition metal compd. is prepd. in an inert liq. medium by rotating a movable body 15 for stirring by means of a magnetic driving device 27 provided with annular magnetic bodies 28 and 29 in this reacting space 7. Thereafter, the filtration space 6 is positioned in the lower space by revolving the revolutional reactor and filtration is performed by a filtration plate 4, and then, washing and drying are performed to obtain the objective powdery product of the catalyst solid.