We consider strictly executable encodings for a classical information transmission assisted by a quasi-Bell state and show that an optimum encoding to maximize Holevo information corresponds to I, X, Z, and XZ when a priori probability distribution of classical information is assumed to be uniform. Then using the encoding, we compute the Holevo capacity by optimizing the probability distribution and clarify that assistance of the quasi-Bell state is effective when the transmissivity of the channel is greater than 0.75.