Problem: To design a gantry-type beam delivery system for heavy-ion therapy beams. In order to allow for installation of the gantry in hospital environment, small overall gantry size and low weight are required. The isocentric configuration is to be preferred. Solution: A new gantry concept with oblique output beam is introduced. Transformations between the treatment angle, angles of gantry and patient table rotations are derived. Results and Conclusions: The isocentric gantry with the overall radius of 2.8 m is feasible using the 60 degrees output beam. The oblique output beam imposes some restrictions on gantry capabilities, which has to be carefully evaluated by radiation oncology community.