The factors controlling the concentration dependence of the electrolyte conductance in supercritical water (SCW) have been analyzed to establish a reliable method to obtain the limiting conductance Lambda(o) and the association constant K-A from a set of conductance data in a moderate concentration range. The conductance measurements should be carried out in such a moderate concentration range because of experimental difficulties. The equations of conductance (EOC) used here are the Fuoss-Chen-Justice (FCJ), the Fuoss-Hsia-Fernandez-Prini, and the Shedlovsky equations. We have shown that two-parameter (Lambda(o) and K-A) fitting methods provide more reliable results than three-parameter methods in a moderate concentration range, and that the three equations give similar fitting results. We have examined what contributions are made by individual terms in the FCJ equation at a moderate concentration in SCW and found that the higher-order terms in concentration nearly cancel each other despite the large individual contributions. This cancelling out is the reason why the EOCs used here work well at a moderate concentration in SCW. Using the method established here, we analyzed the conductance data for LiCl, NaCl, and KCl in SCW and subcritical water and clarified the general trends of the temperature and density dependence of the limiting conductance and the association constant for the simple electrolytes.