To analyze in-cylinder soot formation process, we have newly developed a fast gas sampling valve. The valve is directly actuated by piezo electric device and it achieves the minimum opening duration of about 0.2 ms. The other features, to avoid the growth or the coagulation of the captured soot, the valve has ability which dilutes the sampled gas by nitrogen at the tip of the valve. There was a point in the settlement of the dilution rate to make the analysis succeed. We have applied the valve to our homogeneous-charge-compression-ignition engine, and measured the changes of particle size distribution by using a scanning-mobility-particle-sizer. The measured results clearly show the soot formation process from the soot precursor appearance to the coagulation of them.