Sharkskin-type melt fracture is generally produced by a discontinuous boundary condition at a die exit. In order to investigate this flow instability, a shear flow having a negative pressure gradient at the exit was generated by a wedge-shaped slider. A high resolution differential interference. contrast microscope was used for simultaneous observation of the inside and outside of the flow cell, which employed linear PDMS. At the cell exit, the spatial distribution and the temporal change in velocity were measured under sharkskin-like instability by use of a particle tracking method and laser Doppler velocimetry. A slip flow and a stick flow appeared alternately at the exit of the sliding plate; the slip flow is associated with cavity penetration from the cell exit. This inner instability was found to be associated with the formation of valleys and ridges on the outer free surface. At the cell exit, a slip flow appeared with a valley, and a stick flow appeared with a ridge. The penetration length of the cavity increases with sliding-plate speed, and this increase in penetration length induces flow stagnation upstream of the cavity. This stagnation relaxes high shear upstream of the cavity and suppresses the increase in penetration length of the cavity. Except for incipient instability, the penetration length has a linear relation with the wavelength of the outer ridges. A close relationship is found to exist between free surface roughness and inner flow instability. (C) 2003 The Society of Rheology.