We examined a relationship between rupture propagation directions and the distribution of fault strength by analyzing seismological data from the earthquake swarm on the Yamagata-Fukushima border, NE Japan. This earthquake swarm exhibits a distinct hypocenter migration behavior and was estimated to be triggered by upward fluid movement after the 2011 Tohoku-Oki earthquake. We utilized the dense nationwide seismic network in Japan to estimate apparent source time functions of >1500 small earthquakes (MJMA ≥ 2). We found clear directional dependences of the peak amplitude and the pulse-width in the apparent source time functions, suggesting the earthquake rupture directivity, for half of the earthquakes. Rupture directions mostly avoid the directions of the hypocenter migration. The difference between the microscopic and macroscopic propagations of rupture might be explained by the spatial variation in the fault strength affected by pore pressure along the fault; ruptures of each earthquake are hindered from developing toward the region with higher fault strength ahead of the pore-pressure front. Estimates of stress drop systematically increased on taking the effects of rupture directivity into account. We observed a temporal increase in stress drop from 3 MPa to 10 MPa during the pore-pressure migration.