An improper connection determination method includes a closed circuit forming step, a current detection step and a determination step. In the closed circuit forming step, a closed circuit from one of at least two contacts in a telephone connection terminal of a communication apparatus to another one of the at least two contacts in the telephone connection terminal is formed. In the current detection step, a current flowing in the closed circuit, when the closed circuit is formed in the closed circuit forming step, is detected. In the determination step, it is determined that the telephone line is improperly connected to the telephone connection terminal when the current is detected in the current detection step.