Studies on soft robotics have explored mechanisms of switching the stiffness of a robot structure. The positive-pressurization method has attracted significant attention in recent years as it can remove the limit on the driving pressure. Moreover, it can change the shape holding force according to the value of internal pressure. In this study, a variable stiffness mechanism, consisting of 3-axis rotary ball joints and a single chamber, is devised via frictional force using positive pressure. Based on the theoretical model of the torque required to hold the joint angle, measurement of torque, and FEM analysis of torque, it was confirmed that the value of the holding torque in the roll direction was approximately π/2 times larger than that in the pitch direction for each value of the internal pressure.