PURPOSE: To measure viscosity a wide range with high accuracy by applying a constant differential pressure system when the viscosity of liquid specimen is equal to or above a preset value, and adopting a constant flowrate system when the viscosity is less than the preset value.

CONSTITUTION: Measurement fluid is sent to a measurement capillary on the operation of a determination pump. The pressure drop of the liquid at both ends of the capillary in this case is sent to a differential pressure sensor 1 through a pressure transmission tube via a pressure receiving diaphragm for measuring differential pressure across the capillary. Also, a differential pressure signal from the sensor 1 is transmitted to a differential pressure control device 8, and the speed of a servo motor 6 is determined, so as to keep the differential pressure at the preset value. In addition, a signal selector 9 determines whether a signal from the device 8 or from speed setting unit 10 are transmitted to a servo motor speed control device 11. When a signal set in the device 11 is selected, the servo motor 6 begins to rotate at constant speed, and thereby, the viscometer becomes a viscometer of constant flowrate system. Also, differential pressure measurement value and speed measurement value are transmitted to a viscosity calculation circuit 13 for conversion into a viscosity value.