Abstract: In a central transmission device of a four-wheel steering system, a turning output unit includes an output shaft extending rotatably through a rear wall of a housing for connection with a rear wheel turning mechanism, and a rotary disc coaxial and integral with a front end of the output shaft. The rotary disc has a radially extending slide groove. A turning input unit includes an input shaft extending rotatably through a front wall of the housing for connection with a front wheel turning mechanism, and a cam protrusion disposed eccentrically at a rear end of the input shaft. The cam protrusion extends into the slide groove and drives the rotary disc. The rotary disc turns reciprocally within a limited angle when the cam protrusion rotates one revolution.