An apparatus for measuring the viscosity of a viscous substance as a sample to be measured has a tuning fork type vibrator having two vibrating plates opposing to each other at a distance. These vibrating plates are vibrated at equal frequencies in inversed phase relation to each other by means of a variable vibration generator. The vibrating members have thin plate-like sensitive members provided at their respective lower ends. An amplitude obtained by immersing the sensitive members in a viscous substance and vibrating the former is detected by a pickup device to detect a change in amplitude according to the viscous resistance of the viscous substance thereby to measure the viscosity thereof. The viscosity measuring apparatus has means for regulating the frequency of the vibration generator so as to vary the frequency according to need. Thus, when the relationship between viscosity and responsive vibration no longer maintains a substantially linear relationship at a predetermined driving frequency, the setting of the driving frequency is renewed, thereby allowing viscosity to be measured over a wide range and with a high accuracy.