The development of a large-aperture (120–180 mm) dipole magnet is proposed in the framework of the CERN-KEK cooperation program. The application target is the D1 magnet (separation dipoles) replacement for the HL-LHC (High Luminosity—Large Hadron Collider) upgrade. The Cos-theta type coil cross section and the shell-based structure are adopted in the conceptual design of this magnet. The nominal field is estimated to be 6–10 T at 1.9 K with a 30-mm-width coil arranged in two layers. The candidates of superconductor are $\hbox{Nb}_{3}\hbox{Al}$ , $\hbox{Nb}_{3}\hbox{Sn}$ and Nb-Ti. We present the analytical estimation of the key parameters of this magnet and the simulation of the actual design which includes the field quality, the stray field, and the stress distribution in the coil.