Disclosed is a device for measuring the concentration of the particles contained in a fluid. The device comprises a control volume body having a predetermined effective volume. An inlet path is formed at an end of the control volume body to feed the fluid into the control volume body therethrough. An outlet path is formed at another end of the control volume body to discharge the fluid from the control volume body therethrough. Measuring instruments are provided at the inlet path and the outlet path to emit electrical signals when the fine particles pass through the inlet path and the outlet path. A computing machine receives the electrical signals transmitted from the measuring instruments, and then computes the number and the concentration of fine particles contained in the control volume body. The device is easily integrated with Micro-TAS (Total analysis System).