A plurality of delay stages (21) are connected in cascade, each delay stage having a construction in which either one of a path of a delay element (17) utilizing the propagation delay of a gate array and a path (18) not passing through the delay element is selected by a path selector (19). Each bit of control data is used to control the path selector of the corresponding delay stages. Composite delays are measured for all combinations of such paths, control data (D C ) which provides a measured composite delay closest to an intended delay corresponding to each set data (D L ) is determined and is prestored in a main conversion table (22). A prediction is made, through calculation, as to a delay for each control data when ambient temperature rises △T°C from a temperature T O at which the main conversion table was produced, the thus predicted delay is used to determine control data which provides a predicted delay closest to an intended delay for each set data, and the relationship between the control data and the set data is prestored in a corrected conversion table (31, 32). Ambient temperature is detected by temperature detection/control circuit (36). When the difference △t between the detected temperature and the temperature t O is △t