Since exoskeletons show potential for rehabilitation therapy, many scientists have been designing upper extremity exoskeletons. Unfortunately, few have successfully provided a shoulder exoskeleton for severe impairment. Toward Brain-Machine-Interface (BMI) rehabilitation robot therapies for severe upper extremity impairment, this paper introduces a shoulder exoskeleton robot with a modular joint and an off-board modular actuator. We applied a Modular Exoskeletal Joint (MEJ) to a shoulder exoskeleton that was driven by Pneumatic Artificial Muscles (PAMs) transmitted by a Bowden cable. Our objective is generating passive movements triggered by BMI. Since large torque has to be generated for assist a whole arm, we newly designed a more powerful Nested-cylinder PAMs (NcPAMs) than our previous work. As a proof of the concept of the mechatronics design, we show the tracking performance of the periodic trajectory of a joint angle with both human and mannequin arms as simulated impairments. Our result shows that the tracking error is sufficiently small in all of the conditions and that our developed shoulder exoskeleton is an adequate substitute for flexion/extension movements.