We are planning to carry out a high-resolution spectroscopy of Ξ hypernucleus at the J-PARC K1.8 beamline, which provides a highintensity K⁻ beam (J-PARC E70 experiment). The high-resolution spectroscopy aims to be realized by introducing a new magnetic spectrometer S-2S and an active fiber target AFT. In this article, the role of the AFT in this experiment and its development status are described.