This paper deals with the optimization of a heat insulator in a double lining for icicle prevention, in which a heat insulating material is sandwiched between the outer and inner linings. A tunnel model using a non-steady state heat convection/conduction is proposed for a tunnel-ground with adiabatic treatment. The calculated values well agreed with the observed values of actual tunnels. Then using the tunnel model a procedure to select the material and the thickness of a heat insulator is proposed. Thus a heat insulator to be provided in a newly constructed tunnel with an adiabatic double lining can be designed reasonably and economically on the basis of the periodicity of an atmospheric condition and the thermal conditions of a tunnel-ground.