PURPOSE: To enable measurement of viscosity of a fluid by a simple and compact installation by rotating a rotary drum containing the fluid the viscosity of which is to be measured and by measuring the rotational torque of the drum and the number of rotations thereof.

CONSTITUTION: An apparatus is constructed of a rotary drum 22 having a port 21 enabling putting-in and taking-out of a fluid to be measured in a part on one side, a torque detector 23 and a rotation detector 24 for the rotary drum 22, and a motor 25. The relationship between the rotational torque of the rotary drum 22 and the number of rotations thereof varies with a change in viscosity. Accordingly, the maximum value of the torque and the number of rotations at the time when the torque is the maximum or the number of rotations at the time when the torque lowers are read and measured from curves obtained by the torque detector 23 and the rotation detector 24, and thereby the viscosity of the fluid being a sample of which the viscosity is to be measured can be determined by calculation. According to this constitution, the viscosity of the fluid such as fluidized dredged earth and sand containing solids and having a high water content ratio can be measured by a simple and compact installation.