EM (electromagnetic) field around a relativistically ac-celerated charged particle is known to be squeezed longi-tudinally. This behavior is called the Lorentz contraction, and no inconsistent phenomena have been found. How-ever, an experiment has not directly confirmed the Lo-rentz contraction of the EM field. The first direct observa-tion of the Lorentz contraction of the EM field was recent-ly performed using an electron linac at the University of Osaka[*]. The electric (Coulomb) field around a sub-picosecond electron beam with an energy of 35 MeV was measured by an electro-optic (EO) sampling method. A single-shot electric field measurement system was devel-oped using EO sampling and an echelon mirror. A modu-lated laser light due to the Pockels effect was decoded into a spatio-temporal image of the electric field, and the Lo-rentz contraction was directly confirmed. This ultrafast measurement technique can help longitudinal diagnostics of a charged particle beam. This presentation will report ultrafast visualization of quasi-three-dimensional (trans-verse and longitudinal) electric fields of a relativistic elec-tron beam and their evolutions.