In the research field of virtual and augmented reality (VR/AR), a highly stretchable sensor fiber that can easily be integrated into a sensor glove or finger cot has been required for detecting human hand motions. In this study, we propose a fiber-type highly stretchable strain sensor by slitting an elastic polyurethane (PU) film coated with a conductive polymer of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). A 100-μm-thick PU film coated with 100-nm-thick PEDOT:PSS was slit by a laser cutter with a spot diameter of 125 μm. The width of the sensor fiber was only 214 μm, which is equivalent to a conventional thin 205-μm-diameter fishing line. To investigate the strain sensitivity of the PEDOT:PSS-coated fiber sensor at high stretching (>30%), we evaluated the gauge factor as a function of the concentration of ethylene glycol (EG) in PEDOT:PSS solution. The PEDOT:PSS strain sensor fiber with an EG concentration of 0.75% under 37.5% tensile stretching exhibited the highest gauge factor of 0.16. Finally, the sensors were integrated with thin woven fibers using a flat weave method, and the bending movement of a human finger was detected. The manufacturing method proposed in this study is expected to contribute to the realization of a stretchable strain sensor by the use of a wearable polymer die coating and a slitting apparatus.