A new observing apparatus and a computer-aided image processing system have been developed for peel testing. The observing apparatus detects an image of the peel boundary at the interface of a pressure-sensitive adhesive ( psa) tape with a transparent plate, and peeling speed and stringiness width at constant load can be measured by the image processing system. Observation on a crosslinked polyacrylic acid psa shows that the peel stringiness pattern has a sawtooth appearance at the leaving edge. The difference in peel speed betweeb rubber and polyacrylate adhesives cannot be detected under conditions of a constant load. On the other hand, observation of the peel stringiness width reveals differences between the two. The newly developed apparatus and image processing system thus allows the peeling properties of psas to be examined.