Poly (ethylene terephthalate) (PET) fiber was heated by carbon dioxide laser radiation in the continuous drawing process. By this procedure, the position of the deformation region could be fixed precisely in the air. The location of neck-like deformation fluctuated within a range of about 0.2 mm for draw ratios of 4.0 to 4.5 and within a range of about 0.5 mm for draw ratios of 5.5 to 6.0, but the location fluctuated over a range of 1.0 mm for a draw ratio of 5.0. Fiber diameter profiles, which were used to calculate Hencky strain rate profiles and apparent elongational viscosity profiles, were obtained from high-speed video camera images of the deformation region. Regardless of draw ratio, apparent elongational viscosity exhibited almost the same minimum value. Apparent elongational viscosity is much lower than the value obtained by measurement at a low, constant strain rate, but the elongational stress acting at the point where apparent elongational viscosity begins to increase steeply (Hencky strain of about 1.0) is of the same order of magnitude as the reported value. For draw ratios less than 5.0, the temperature where apparent viscosity is lowest is about 100 to 120°C, which corresponds to the flow temperature of amorphous PET, whereas for draw ratios exceeding 5.0 the temperature where apparent viscosity is lowest is about 80°C, which corresponds to the glass transition temperature. Thus, the former corresponds to so-called neck-like deformation typically observed in high-speed spinning, and the latter corresponds to necking typically observed in cold drawing. These two types of deformation appear in turns for a draw ratio of 5.0, and therefore the location of the deformation region fluctuates greatly. This measurement system can be used as a high-strain-rate elongational rheometer for analyzing practical polymer processing systems, which can easily measure the precise on-line deformation history with a time resolution in the μs level.