It was theoretically predicted that, when the electron pulse length comes into the sub-picosecond range, transverse motion of the electrons is strongly coupled with the longitudinal one and makes significant effect on the pulse shape. In the experiments, a fine dip structure was created on the electron bunches circulating in a storage ring by a so-called laser bunch slicing technique and then the evolution of the structure was observed through the spectrum of the coherent synchrotron radiation (CSR). When the ring was operated in a low-alpha mode, the shape of the dip structure was oscillating with the transverse betatron frequency, which clearly indicates the existence of the longitudinal-transverse coupling effect.