A ferroelectriclike dielectric response was observed in a field-effect transistor using oriented bis(ethylenedithio)tetrathiafulvalene-tetracyanoquinodimethane crystals. Phase transitions at 285 and 320 K were clearly observed in the temperature dependence of field-effect electron mobility. The phase transition at 320 K corresponds to the metal-insulator transition previously reported in a bulk crystal. On the other hand, the field-effect electron and hole mobilities exhibited an abrupt increase at 285 K, which had not been discovered by other physical measurements in the bulk crystal and is nonetheless sufficiently stable and reproducible. In addition, the abrupt increase in carrier mobilities was clearly correlated with the decrease in the dielectric response. The temperature variation in difference hysteresis curves demonstrated the feature of ferroelectric transition.