Abstract: It is an object of an aspect of the present invention to provide (i) transformants, whose proliferation depends on phosphite, of various species of organism and (ii) a method for detecting the presence of a reduced phosphorous compound with use of such a transformant. Use is made of a transformant which is defective in functions of a gene encoding a phosphate transporter protein and a gene encoding a phosphate ester transporter protein and into which a gene encoding a hypophosphite transporter protein is introduced, a signal peptide of a hypophosphite binding protein being substituted with a signal peptide derived from a host or a species of organism closely related to the host.