A plant monitoring and control system comprising a plurality of monitoring apparatuses (3; 3a, 3b) for monitoring and controlling states of a plurality of plant equipment through a second communication network and a maintenance tool for maintaining the plurality of monitoring apparatuses (3; 3 a, 3b) through a first communication network, wherein at least one of the monitoring apparatuses has: an abnormality detecting unit for detecting an abnormality of a self apparatus, wherein said self apparatus is the monitoring apparatus provided with the abnormality detecting unit; a second communication unit for receiving state information as information regarding the states of the plurality of plant equipment from the plurality of plant equipment through the second communication network; a plant information forming unit for forming plant information as information regarding the whole plant equipment on the basis of the plurality of state information; a first communication unit for transmitting the plant information to the maintenance tool through the first communication network; and an abnormal-case control unit constructed in such a manner that when the abnormality detecting unit detects the abnormality, a substituting request for requesting a substitution of a process of the self apparatus is transmitted to the other monitoring apparatus among the plurality of monitoring apparatuses (3a, 3b); characterized in that (a) when a response to the substituting request is affirmative, all tasks which are operating in the self apparatus are stopped by the abnormal-case control unit, and (b) when the response to the substituting request is negative or when the response to the substituting request is not received, the first communication unit and the second communication unit are instructed to transfer the state information received by the second communication unit to the maintenance tool, and the abnormal-case control unit stops only a task in which the abnormality occurred in the self apparatus (3 a), wherein, when the substituting request is transmitted to the other monitoring apparatus, the abnormal-case control unit instructs both of the first communication unit and the second communication unit so as to transmit the substituting request to the other monitoring apparatus. an abnormality detecting unit for detecting an abnormality of a self apparatus, wherein said self apparatus is the monitoring apparatus provided with the abnormality detecting unit; a second communication unit for receiving state information as information regarding the states of the plurality of plant equipment from the plurality of plant equipment through the second communication network; a plant information forming unit for forming plant information as information regarding the whole plant equipment on the basis of the plurality of state information; a first communication unit for transmitting the plant information to the maintenance tool through the first communication network; and an abnormal-case control unit constructed in such a manner that when the abnormality detecting unit detects the abnormality, a substituting request for requesting a substitution of a process of the self apparatus is transmitted to the other monitoring apparatus among the plurality of monitoring apparatuses (3a, 3b); characterized in that (a) when a response to the substituting request is affirmative, all tasks which are operating in the self apparatus are stopped by the abnormal-case control unit, and (b) when the response to the substituting request is negative or when the response to the substituting request is not received, the first communication unit and the second communication unit are instructed to transfer the state information received by the second communication unit to the maintenance tool, and the abnormal-case control unit stops only a task in which the abnormality occurred in the self apparatus (3 a), (a) when a response to the substituting request is affirmative, all tasks which are operating in the self apparatus are stopped by the abnormal-case control unit, and (b) when the response to the substituting request is negative or when the response to the substituting request is not received, the first communication unit and the second communication unit are instructed to transfer the state information received by the second communication unit to the maintenance tool, and the abnormal-case control unit stops only a task in which the abnormality occurred in the self apparatus (3 a), wherein, when the substituting request is transmitted to the other monitoring apparatus, the abnormal-case control unit instructs both of the first communication unit and the second communication unit so as to transmit the substituting request to the other monitoring apparatus.