In the present study, in situ measurements of applied torque and compressive load were conducted during high-pressure torsion (HPT) on Ti-23%Nb-0.7%Ta-2.0%Zr-1.2%O (in at %), Gum Metal, by using four active strain-gage method. The shear stress was then calculated from the measured torque. The in situ measurements revealed that the maximum shear stress reaches ∼2 GPa during HPT. This value is comparable to the ideal shear strength of Gum Metal, which was reported as ∼1.8 GPa from experiments using single crystals. The deformation mechanism strongly depends on body-centered cubic (bcc) phase stability at an early stage of HPT straining, where the shear stress is well below the ideal shear strength. On the other hand, the deformation mechanism may be insensitive to the bcc phase stability at a later stage of HPT straining, where plastic deformation occurs at a strength close to the ideal shear strength.