A particle formation diagnosing system monitors the condition of particle formation in each of reaction chambers 1 of a semiconductor device fabricating line on the basis of data provided by a pressure measuring device 2a, a temperature measuring device 2b and a differential mobility analyzer 3 combined with each reaction chamber 1. The operation record recording unit 4a of the computer system 4 records data on the relation between the values of the operation parameters of each reaction chamber 1 and the amount of particles formed in the reaction chamber. Then, the operating condition determining unit 4b determines the optimum values for the operation parameters of the reaction chamber which will reduce the possibility of particle formation to the least possible extent on the basis of the data recorded by the operation record recording unit 4a.