Provided are a method, upon radical copolymerization of a vinyl-containing porphyrin compound and a radically polymerizable monomer, thereby obtaining a porphyrin-nucleus introduced polymer, of controlling reactivity of the porphyrin-nucleus introduced polymer with a metal by selecting the radically polymerizable monomer; and a method for measuring a trace metal by measuring a change in absorbance caused by a complex forming reaction between a porphyrin-nucleus introduced polymer, which has been obtained by radical copolymerization of a vinyl-containing porphyrin compound and a radically polymerizable monomer, and a heavy metal ion, the radically polymerizable monomer being selected so as to heighten reaction selectivity of the porphyrin-nucleus introduced polymer to a metal to be measured. The present invention makes it possible to control the properties or reactivity of the porphyrin-nucleus introduced polymers, which have been obtained from easily available porphyrin compounds without any modification, and by making use of this technique, to provide a highly sensitive method for measuring heavy metals.