The π-d molecular conductor λ-(BETS)2FeCl4 consists of the conducting π-electrons in BETS molecules and the magnetic d-electrons in FeCl4 -. Thanks to the strong interaction between πand d-electrons, λ-(BETS)2FeCl4 has a fascinating temperature(T)-magnetic field(B) phase diagram including a paramagnetic metal phase, an antiferromagnetic insulating (AFI) phase and a fieldinduced superconducting phase [1-2]. However, the ground state of the AFI phase is still under debate [2] . It was previously

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... elieved that the AFI phase is due to the antiferromagnetic long-range order of the d-electrons. However, recent heat capacity measurement observed a Schottky anomaly below TN, and its origin was explained from the contribution of paramagnetic d-electrons. This suggests that the d-electron remains paramagnetic below TN, and only the π-electron becomes antiferromagnetic and localized [3] . Nevertheless, this explanation is not consistent with the existence of strong π-d interaction, and contradicts with previous electron spin resonance (ESR) studies [4] [5] [6] . In addition, anomalous dielectric behavior due to the metastable state of the π-electrons was reported for the AFI phase [4] . Hence, to have a more microscopic information of the AFI phase, we are studying λ-(BETS)2FeCl4 using ESR spectroscopy. We previously presented [7] that the easy-axis of λ-(BETS)2FeCl4 is changing with temperature. In order to study the relation between the change of the easy-axis and the external field, we have investigated the field-dependence of antiferromagnetic resonance (AFMR) using highfield ESR spectroscopy. When the magnetic field is applied parallel to the easy-axis, the easy-axis mode of AFMR is observed below 5 T, however, the easyaxis mode of AFMR changes to the hard-axis mode of AFMR above 5 T as shown Fig. 1 (conventional AFMR mode is shown in the inset of Fig. 1 ). This suggests the change of the easy-axis occurs above 5 T. In addition, the change of permittivity was reported around 4-6 T [4], which suggests that the magnetic properties of λ-(BETS)2FeCl4 should be strongly connected with the electronic state of the π-electrons. Therefore, the change of the easy-axis is supposed to be due to the metastable state of the π-electrons. Moreover, splitting of AFMR signals is observed above 5.9 T (as shown Fig. 1 ), this behavior suggests two different environments of Fe spins exist.