In this study, the lattice compression properties of monoclinic HfO2 (m-HfO2) at room temperature are investigated by performing high-pressure powder x-ray diffraction experiments. The lattice constants are precisely determined from the Rietveld analysis. The linear compressibility of each crystal axis a, b, and c is estimated as 10.0x10(-3), 3.8x10(-3), and 12.7x10(-3)/GPa, respectively, indicating a strong anisotropy. The equation of state for m-HfO2 is obtained using the third-order Birch-Murnaghan equation based on the volume and pressure data. The obtained unit cell volume (V-0), bulk modulus (B-0), and its pressure derivative (B-0 ') at ambient pressure are 138.3(1) angstrom, 195(2) GPa, and -5.4(5), respectively. The negative value of B-0 ' implies the bulk modulus softening with an increase in pressure. The fact that the elastic softening has also been observed in m-ZrO2 suggests that the abnormal behavior is a characteristic of the monoclinic structure (P21/c). The transition to the orthorhombic phase is observed at approximately 6.4GPa. The transition is sluggish, and two phases coexist at a pressure of up to 10GPa.