An image processing method for the analysis of a turbulent structure was devised. It was applied to an impulsively started axisymmetrical jet growing into an otherwise quiescent fluid. The development of the statistical average contour and the growing behavior of the jet were clarified. Simultaneous measurements of the concentration and the velocity at a fixed position were carried out and the transient phenomenon at the position was made clear. It is confirmed that the image processing method developed here is effective for the analysis of the spatial structure of turbulence in an unsteady flow field.