Abstract: An apparatus for measuring viscosities of liquids has a narrow tube for a test liquid to be flowed. A differential pressure is generated between upstream and downstream of the test liquid. A differential pressure is measured at each divided time period for a total pass-by period which is a time for the test liquid to pass from a first to a second positions in the narrow tube. An integrated quantity obtained by determining a product of each divided time period and the differential pressure over the total pass-by period. Viscosity of the test liquid is obtained by comparing the integrated quantity with a corresponding integrated quantity previously determined in a reference sample liquid of known viscosity.