We developed a theoretical method that can systematically treat the phase-matching condition of nonlinear infrared or Raman measurements. This method might be a rational tool for the analysis of observed signals from various directions based on the response Function especially under non-impulsive excitation. Model calculations of the third-order nonlinear IR spectroscopy for a Brownian harmonic oscillator system are performed. We discuss the variation of signals that depend on the observation direction and the temporal width of excitation pulse in comparison with the behavior within the impulsive limit approximation.