The three-dimensional (3D) plasma response to the magnetic field structure is studied for high-β plasmas in the Large Helical Device (LHD). For the vacuum field in the LHD configuration, the boundary of the plasma is defined by the last closed flux surface. However, for high β, the magnetic field structure is changed by the 3D plasma response and the plasma boundary is shifted in the radial direction. The radial electric field, Er, to identify the plasma boundary, is measured in the peripheral region. The radial electric field shear from negative to positive appears in the region and this suggests the boundary between open and closed field lines. The position of the Er shear is always outside the vacuum boundary. A 3D magnetohydrodynamic modelling predicts the expansion of the stochastic region by the 3D plasma response. However, since the connection length of the stochastic field lines is larger than the electron mean free path, a confinement is expected in the region. In such a case, the edge of the stochastic region can be defined as the effective plasma boundary. The position of the appearance of the strong Er shear is almost comparable to the edge of the stochastic region of the modelling. That is, the 3D plasma response is identified in LHD experiments.