A driving posture adjusting device comprises a slide rail (39) disposed in a forwardly raising posture on a floor portion of a vehicle body; a seat slider (44) disposed in a lower front portion of a seat cushion (35) of a driver seat and slidably supported along the slide rail (39); a seat driving mechanism (40) for moving the driver seat in forward and backward directions by driving the seat slider (44); a guide plate (51) formed with a guide groove (53) of a predetermined path and fixed on the vehicle body; and a guide member (52) provided in a rear portion of the seat cushion (35), the guide member including a guided portion (52) supported along the guide groove (53), where a rear end portion of the seat cushion is raised by sliding the guided portion along the guide groove of the guide plate in response to an operation of moving the driver seat toward a front side of the vehicle body by driving the seat slider by the seat driving mechanism.