The present invention relates to a driving speed control device for an agricultural work vehicle, and a driving speed control method for an agricultural work vehicle, the device comprising: a confirmation unit for confirming set transmission information about a driving device of an agricultural work vehicle, and confirming a transmission mode for controlling an engine and a transmission of the driving device; an acquisition unit, which confirms, from the set transmission information, set revolutions per minute (RPM) relating to RPM of the engine when the transmission mode is an RPM designation mode, so as to acquire transmission information on the basis of the set RPM; and a control unit for controlling the driving device by using the acquired transmission information according to the transmission mode.