In a plant equipment comprising apparatus of a plurality of systems and control means for switching the systems and changing capacities of the apparatus, an apparatus of each system bas a rated capacity which is 1/N of a capacity of the whole equipment, and the plant equipment is constituted of (N+1) systems. In conditions of normal operation, all the (N+1) systems are operated at capacities less than the rated capacity through the control means. During normal operation, when one system is inspected, or when an abnormality occurs in one system, this system is stopped by the control means. Capacities of apparatus in N of the systems which are operated are increased to the rated 1/N capacity through the control means, and operation is continued with a structure of 1/N capacity.times.N systems. When the maintenance of apparatus in the stopped system is finished, the stopped system is restarted, so as to operate all of the (N+1) systems at capacities less than the rated capacity again.