Microseismic monitoring is a crucial technique for successful hydraulic stimulation/fracking for various types of subsurface development. Knowledge about the geometry of the fracture is also essential for a better understanding of many phenomena in reservoirs. Fracture orientation can be estimated by the seismological method under very good monitoring situation. However, we often face difficulty in constraining the focal mechanism of microseismicity from first motion due to the small number of stations. We develop a method to constrain the range of focal mechanisms solutions by introducing in-situ stress information. We can reject candidate focal mechanisms which are consistent with first motion but not consistent with in-situ stress by checking two conditions. 1) consistency between shear vector estimated from in-situ stress and the rake from first motion analysis, 2) state of stress of the fracture and its stability considering the injection pressure. We use field data to demonstrate that this method can significantly constrain the range of focal mechanisms.