[en] We describe a new detector system developed for high-altitude balloon flights to observe the cosmic-ray electrons above 10 GeV. The Balloon borne Electron Telescope with Scintillating (BETS) fibers instrument is an imaging calorimeter which is capable of selecting electrons against the large background of protons. The calorimeter is composed of a sandwich of scintillating optical-fiber belts and lead plates with a combination of three plastic scintillators for shower trigger. The total thickness of lead is 40 mm (∼ 7.1 r.l.) and the number of fiber belts is nine. In each belt, alternating layers are oriented in orthogonal (x and y) directions. Two sets of an intensified CCD camera are adopted for read-out of the scintillating fibers in the x and y direction, respectively. The accelerator beam tests were carried out to study the performance of detector for electrons in 1996 and for protons in 1997 at CERN-SPS. The instrument was successfully flown aboard high-altitude balloon in 1997 and 1998. It is demonstrated by the flight data that a reliable identification of the electron component has been achieved in 10-100 GeV and the energy spectrum has been obtained