We present a common-path phase-shifting interferometer in which photonic crystal polarizers (PCPs) are utilized as a reference mirror and a phase shifter, allowing ultracompact and highly sensitive optics. When a laser beam polarized at $45°$ relative to the optical axis of the PCP-based reference mirror is incident, the polarization component parallel to the optical axis (s-polarized beam) is reflected and used as a reference beam. The perpendicular component (p-polarized beam) passes through the PCP coupled with a quarter-wave plate (QWP) and serves as a probe beam. This beam, with its polarization transformed in the sequence p, right-circular, s, left-circular, and p, irradiates the sample surface twice, doubling the phase change due to displacement of the sample. The probe beam is then retransmitted through the PCP, where it recombines with the reference beam. Four interferogram channels in phase quadrature are generated using a newly developed phase shifter, composed of a QWP and a monolithically integrated array of four PCPs. Preliminary experiments demonstrate that the PCPs perform successfully as a reference mirror and a phase shifter, and that the interferometer has a remarkable displacement sensitivity, as low as $40\mathrm{pm}$ .