This article deals with an experimental observation of the longitudinal vortex pairs being formed intermittently in the stagnant region of an impinging jet from a viewpoint of selective enhancement of jet impingement heat transfer. A two-dimensional submerged water jet issuing from a convergent slot nozzle was made to impinge normally on a solid flat plate. Intermittent generation of the vortex pairs visualized by using a hydrogen bubble technique was recorded by a high-speed video camera. Large-scale turbulent structures were also observed in the stagnant region of an impinging jet with a Laser Doppler Velocimeter. A statistical analysis with conditional sampling was applied to extract each event of vortex-pair generation from the time-series of LDV velocity data as well as the time-series video images of visualized flow patterns. It has been found that the frequency of vortex-pair generation increases with nozzle-to-plate distance up to 6 nozzle widths. This tendency is coincident with that of the enhancement effect of jet impingement heat transfer depending on the jet development distance. It can be conjectured that those longitudinal vortex pairs appear owing to the wave instability of the shear vortex core filaments in the shear layer of the approaching jet.