A biological molecule detecting apparatus capable of highly sensitive measurements is provided. A laser was emitted onto a solution, to impart external force onto free molecules and binding molecules within the solution. The external force inhibited Brownian motion of the free molecules and the binding molecules. The concentration of a detection target substance which is associated the binding molecules can be measured with high sensitivity, by measuring the Brownian motion of the free molecules and the binding molecules within the solution irradiated with the laser.