PURPOSE: To provide method and apparatus for measuring an average current velocity for a relatively long time required for investigating the flow environment at a habitat of underwater organisms.

CONSTITUTION: A measuring apparatus containing a water soluble substance causing variation in the concentration through function of the flow wherein a linear relationship is satisfied between the temporal variation rate of the logarithmic concentration and the current velocity is disposed at a measuring point. The concentrations Cin (O), Cin (T) of the water soluble substance are measured at the start and the end of measurement and substituted into a following formula to determine an average current velocity. 1/T∫0Tudt=u0-1/ (βT) log [{Cin(T)-Cout)/(Cin(0)-Cout}]. The measuring apparatus comprises a solution chamber 6 encapsulating the water soluble substance being mixed uniformly with water of dye, for example, with a thin flow-out pipe 1 facing the water flow to induce pressure drop through entraining or separation being coupled with the upper part of the solution chamber 6. A water inlet 7 filled with a flow braking material 8 for damping the kinetic energy through loss of viscosity is provided on the bottom. This constitution realizes investigation of flow environment through a convenient measurement at low cost.