In SPring-8, the 8 GeV compact XFEL "SACLA" is under commissioning. A single bunch of electrons is compressed down to about 30 fs for brilliant SASE X-ray lasing. It is an important key of stable lasing to investigate the longitudinal phase space and the sliced emittance of a lasing part of the bunch by using a transverse RF deflector. We developed a high gradient C-band deflecting structure operated at 5712 MHz for the bunch diagnosis with a resolution on a femtosecond scale in a limited space in SACLA. The backward travelling-wave of the HEM11-5π/6 mode is excited in the cylindrical structure periodically loaded with racetrack-shaped irises. The featuring irises suppress rotation of the deflection plane and generate strong cell-to-cell coupling for stable resonance. Two 1.8m-long structures were fabricated and installed in SACLA. They successfully generated a deflection voltage of 60 MV and pitched the bunch at the zero-crossing RF phase.