There is an ongoing plan to upgrade the 25 T cryogen-free superconducting magnet (25T-CSM), at the High Field Laboratory for Superconducting Materials (HFLSM), IMR, Tohoku University, to a 30T-CSM by replacing the existing Bi2223 insert coils with REBCO coils. For the REBCO coils of the 30T-CSM, we will adopt a robust coil concept, which consists of two tapes co-wound in a face-to-back configuration with thin FRP plates glued on the edges. To confirm the effectiveness of this concept, we performed hoop stress tests on full-scale-size four stacked pancake coils fabricated with Fujikura EuBCO tapes at conduction-cooled 20 K under a background field of 11 T. The inner and outer diameters of the pancake coils were 68 and 268 mm, respectively. The averages of the measured strains on the outermost winding were 0.17-0.24% at a hoop stress of 400 MPa estimated by the BJR relation. The I-V properties of the coils indicated no anomalous voltage, implying that our coil structure was robust against large electromagnetic stress.