An increase of the coupling resistance of an ICRF 2 × 2 phased loop antenna array after H-mode transition was first observed in JT-60. The coupling resistance of the ICRF antenna decreases after H-mode transition in the (0,0) antenna phasing (toroidal phase difference), but it increases in the (π,0) antenna phasing (poloidal phase difference). This different behaviour can be explained by antenna-plasma coupling theory, taking the antenna phasings and the electron density profiles into account. The coupling resistance can be kept constant after H-mode transition by adjusting the toroidal wave number spectra of the antenna currents.