To suppress impulse noise on lofargram, we have proposed a smoothing method using morphological opening filter on time-axis. The computer simulation results have shown effective improvements of processing gain and time-resolution as compared with that of averaging procedure. However, same frequency components will not be maintained in target signal at any time. In this case, morphological opening filter on time-axis removes the parts of the target signal in which the frequency components vary with time. It is a disadvantage of this method. In this paper, morphological opening filter using structuring element on time-frequency space is tried to smooth the lofargram. Two types of the sound data are in use to verify this method. The firsts are simulated on computer. The second data are measured in practice. The simulation results in this paper show effective improvements of processing gain. Then, the lofargram outputs of both data can visualize the target signals, the frequency components of which vary with time. We confirm that the smoothing method using the structuring element on time-frequency space outweighs the disadvantage of that on time-axis.