To provide a lead concentration determination reagent which realizes accurate, high-sensitivity, and simple-manner determination of the lead ion concentration (also referred to simply as “lead concentration”) of a sample solution in the presence of calcium ions, and a lead concentration determination method employing the reagent. The lead concentration determination reagent contains (A) a water-soluble porphyrin derivative or a salt thereof, (B) at least one member selected from among polyacrylamide, polyvinyl alcohol, and polyethylene glycol, and (C) a calcium-ion-supplying compound.