Standard plastic scintillating fiber cannot detect low-energy β-rays as the cladding prevents them from reaching the fiber core. We developed an outer-layer scintillating (OLS) fiber with a plastic scintillator on the outermost layer for low-energy β-ray detection. The concept of fiber construction is presented. The fundamental optical properties of the OLS fiber, such as the emission spectrum, attenuation length, and scintillation decay time, were evaluated. Here, Ni-63 with a maximum energy of 67.0 keV was used as a low-energy β-emitting nuclide. Simulation studies on the interaction between low-energy electrons emitted from Ni-63 and a single fiber were performed prior to actual measurements. The data showed that Ni-63 can be measured using silicon photomultiplier photosensors in a coincidence mode. The OLS fiber was effective for low-energy β-ray detection.