An interlayer design and test was made to enhance the joining strength of the pressureless sintered sialon ceramic and 40Cr steel. Joining was performed by vacuum brazing using Ag57Cu38Ti5 filler metal. The joint strength was evaluated by four-point bending. A strong interfacial bond of the Ag57Cu38Ti5 filler metal on the sialon ceramic with formation of Ti2AIN, Ti5Si4 and TiAg was obtained at brazing temperatures over 1123 K, which could be weakened by a brazed metal such as Kovar or Ni-15Cr-15Co alloy. The joint strength of sialon ceramic with 40Cr steel can be improved by using a layer of soft interlayer such as Cu with a suitable thickness, particularly by the composite interlayer such as Cu/Nb alloy, Cu/Ta, Cu/Mo etc. The maximum strength of the ceramic/steel joint, 280 MPa, was obtained by using Cu/Nb alloy as interlayer and brazing at 1153 K for 5 min. Finally, we discuss how to design an interlayer in ceramic/metal joining.