To investigate the characteristics of vibration-induced shear resistance reduction (ViSRR) in granular soils, laboratory tests were performed using a modified triaxial apparatus. Monotonic loading and vibration with controlled frequency and acceleration amplitude were superimposed to soil specimens under drained conditions. Tests were conducted for dry and saturated specimens at relative densities of Dr = 35% and 70%. The tests revealed that superimposed vibration causes a reduction of shear resistance in addition to an additional volume change of the specimen. After the termination of vibration, the shear resistance was found to be recovered as the monotonic shearing continued. It was confirmed that the shear resistance reduction was not caused by vibration-induced variation of excess pore pressure in the specimen. For high-frequency vibration with frequency varying between 60 and 120 Hz, the relative shear resistance loss tended to increase linearly with the peak acceleration. Based on the test results, the concept of “vibro-critical state” is proposed to describe the ViSRR of granular soil subjected to monotonic shear and vibration simultaneously.