Abstract: A process for production of l-alkylthiomethanediphosphonic acid compound or 1-arylthiomethanediphosphonic acid compound characterized by removing a thiolate byproduct as an insoluble salt, in the condensation represented by the following formula: ##STR1## wherein R'.sub.1 represents a linear or branched alkyl group having 1 to 6 carbon atoms, R.sub.2 represents an alkyl or aryl group, R.sub.1 represents a pharmaceutically allowable cation, hydrogen atom, or linear or branched alkyl group having 1 to 6 carbon atoms. The present invention significantly improves the yield compared with conventional methods, and simplifies the purification process at the same time, thus is extremely useful in an economical and industrial points of view.