Integral field spectroscopy (IFS) provides a spectrum simultaneously for each spatial sample of an extended 2-D field. It consists of an integral field unit (IFU), which slices and rearranges the initial field along the entrance slit of a spectrograph. We present an original design of IFU based on the advanced image slicer concept. To reduce optical aberrations, pupil and slit mirrors are disposed in a fan-shaped configuration, which means that angles between incident and reflected beams on each element are minimized. The fan-shaped image slicer improves image quality in terms of wavefront error by a factor of 2 compared with a classical image slicer, and furthermore, it guaranties a negligible level of differential aberration in the field. As an example, we present the design LAM used for its proposal at the near-infrared spectrograph (NIRSpec/IFU) invitation of tender for the James Webb Space Telescope (JWST).