We have observed the J = 2-1 rotational emission line of PN toward the low-mass star forming region L 1157 with the Nobeyama 45 m telescope. The PN line has been detected toward the two shocked regions (B1 and B2) caused by the interaction between the blue lobe of the outflow from the protostar IRAS 20386+6751 and the ambient gas, whereas it has not been detected toward the protostar position. The fractional abundances of PN relative to H2 are evaluated to be (2-6) ×10-10 and (3-7) × 10-10 toward B1 and B2, respectively. Detection of PN in the B1 and B2 positions indicates that the formation of PN can be triggered by shocks. It seems most likely that PN is formed in the gas phase from PH3 evaporated from grain mantles, as suggested by the existing chemical model.