Temporal variations of the fault frictional strength was investigated based on the diversity of focal mechanisms in the source area of the Yamagata‐Fukushima border earthquake swarm, a significant earthquake swarm that occurred in central Tohoku, NE Japan, which started just after the 2011 M9.0 Tohoku‐Oki earthquake. The focal mechanisms of events in this swarm activity were determined using P wave polarity data as well as short‐period (1.5–2.5 Hz) waveform data from the direct P wave. The stress field in the source area of this swarm was estimated by applying stress tensor inversions to these focal mechanism data. Based on the estimated stress field, and under the assumption of uniform stress, we calculated relative frictional strengths for individual focal mechanisms. The calculated relative frictional strengths vary over a wide range, but their average value exhibits a characteristic temporal variation, which is at first small, but steadily increases with time for 100 to 150 days, and then becomes approximately constant. We confirmed this characteristic temporal variation of the average relative frictional strength by assuming the stress to be nonuniform. Similar temporal variations of the average relative frictional strength are obtained for even these cases, confirming the variation. The most likely cause for the observed temporal variation of the average relative frictional strength is the temporal variation of the pore fluid pressure in the source area of the swarm, facilitated by the Tohoku‐Oki earthquake and the subsequent fluid diffusion.