The subject of the paper is to analyze time complexity of the minimum modification problem in the Horn clause propositional logic. Given a set H of Horn clauses and a query Q in propositional logic, we say that Q is provable over H if and only if Q can be shown to be true by repeating Modus Ponens among clauses of H. Suppose that Q is not provable over H, and we are going to modify H and Q into H and Q , respectively, such that Q is provable over H . The problem of making such modification by minimum variable deletion (MVD), by minimum clause addition (MCA) or by their combination (MVDCA) is considered. Each problem is shown to be NP-complete, and some approximation algorithms with their experimental evaluation are given.