Two typical flow instabilities of "Shark Skin" and "Melt Fracture" for high density polyethylene were investigated in this report. Experimental measurements were carried out methodically in a wide flow region from the reservoir (entry region) to the die land with a specific slit die. Especially the flow pattern was visualized with a high speed camera and the frequency of the flow instability was analyzed. Corresponding to the visualization analysis, the pressure fluctuation in the slit die and the surface roughness at the die exit were measured simultaneously and these frequency characteristics were analyzed with a FFT (Fast Fourier Transform) analyzer As for "Shark Skin" region, the irregularity of the surface roughness in the broad frequency band was recognized at the die exit. In spite of this surface roughness, the pressure fluctuation and the fluctuation of tire flow pattern at the wall vicinity in the die land were not observed. These results indicate that "Shark Skin" occurs at the die exit. As for "Melt Fracture" region, the periodical oscillation of the flow pattern and the periodical pressure fluctuation which corresponded to the periodical distortion of the extrudate was recognized in the die land. However the periodical oscillation of the flow pattern and the periodical pressure fluctuation could not be recognized in the reservoir (entry region of the die land). These results indicate that "Melt Fracture" is initiated at the die entry and occurs in the die land. Consequently it is clarified that "Shark Skin" occurs at the die exit and the flow instability which lead to "Melt Fracture" is initiated at the die entry and occurs in the die land.