PROBLEM TO BE SOLVED: To provide a molecular weight measuring method and its device for high polymer fluid to inexpensively continuously and stably perform the measurement with small maintenance labor, and to provide a molecular weight control method and its device for high polymer fluid to measure the molecular weight and to keep the molecular weight within a target range. SOLUTION: An experimental formula indicating the quantitative relationship of the plant operation information including the fluid pressure and fluid temperature in moving the high polymer fluid in a channel 12, and the molecular weight of the high polymer fluid is determined in advance, the plant operation information is measured when the high polymer fluid during its preparation is moved in the channel 12, the measured plant information is assigned to the experimental formula to calculate the molecular weight of the high polymer fluid during its preparation. An operating end 31 for operation mounted on a reactor 11 is adjusted on the basis of the obtained molecular weight of the high polymer fluid as a result of the measurement by the high molecular measuring method, and the molecular weight of the high polymer fluid prepared in the reactor 11 is taken in the predetermined range of molecular weight. COPYRIGHT: (C)2004,JPO
