It was theoretically predicted that, when the electron pulse length comes into the femto-second 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 measured through the spectrum of the coherent synchrotron radiation. 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. This understanding will be crucially important for generation and transportation of ultra-short electron bunches in light sources or colliders for high energy physics. In this presentation, the dependency of the CSR signal intensity on the wavelength of the THz CSR and the electron beam current are also reported.