The ferroelectric phase transformation from the tetragonal phase to the orthorhombic phase, induced by an electric field, is demonstrated in a 5%YO1.5-doped Hf0.5Zr0.5O2 epitaxial film which is grown on Sn-doped In2O3-covered (111) yttria-stabilized zirconia by the pulsed laser deposition method at room temperature and subsequent heat treatment. Although X-ray diffraction shows the film to consist of a paraelectric tetragonal phase after the heat treatment, polarization–electric field (P–E) measurements reveal a hysteresis loop attributed to the ferroelectricity. To clarify the discrepancy between the crystal structure and electric characteristics, the crystal structure after electric field loading is determined by scanning transmission electron microscopy and synchrotron X-ray diffraction measurements. Both structural characterizations clearly reveal that the application of an electric field promotes the phase transition from the paraelectric tetragonal phase to the ferroelectric orthorhombic phase. This ferroelectric transition is irreversible, as the ferroelectric phase remains after the removal of the electric field. These results facilitate the elucidation of the mechanism by which ferroelectricity is displayed in HfO2-based fluorite ferroelectric materials and imply unimportance of the orthorhombic phase for as-prepared films.