A mobile pump device, in particular for use on mobile transport means, such as semitrailers, tank trailers, tank semitrailers, tankers and lorries, with a drive motor and with a rotary piston pump, wherein the drive motor comprises a motor shaft for driving the rotary piston pump and a motor housing, wherein the rotary piston pump comprises a housing with a pump housing part and a gear housing part, wherein two openings (4, 5) are formed on the pump housing part, one of which serves as a pump inlet and the other as a pump outlet, and wherein the rotary piston pump comprises at least two rotor units (10, 20) disposed rotatably in a pump chamber in the pump housing part, in order to convey a fluid from the pump inlet to the pump outlet, wherein the two rotor units (10, 20) are disposed on rotatably mounted rotor shafts (11, 21), wherein each rotor shaft (11, 21) is equipped with a rotor gear (12, 22) arranged outside the pump chamber, wherein a drive pinion of a drive shaft of the rotary piston pump is coupled with one of the rotor gears, wherein the rotor gears (12, 22) and the drive pinion are accommodated in the gear housing part, characterised in that the motor shaft of the drive motor comprises a cutout at the front end, in which the drive shaft of the rotary piston pump is received and coupled with the drive shaft, and that the drive motor is connected via a connection flange with a U-shaped cross-section to the housing, wherein the connection flange forms the rear closure cover of the housing and at the same time the front closure cover of the motor housing, wherein the connection flange comprises two flanges running in parallel and spaced apart from one another, which form a surrounding "U" opened outwards. with a drive motor and with a rotary piston pump, wherein the drive motor comprises a motor shaft for driving the rotary piston pump and a motor housing, wherein the rotary piston pump comprises a housing with a pump housing part and a gear housing part, wherein two openings (4, 5) are formed on the pump housing part, one of which serves as a pump inlet and the other as a pump outlet, and wherein the rotary piston pump comprises at least two rotor units (10, 20) disposed rotatably in a pump chamber in the pump housing part, in order to convey a fluid from the pump inlet to the pump outlet, wherein the two rotor units (10, 20) are disposed on rotatably mounted rotor shafts (11, 21), wherein each rotor shaft (11, 21) is equipped with a rotor gear (12, 22) arranged outside the pump chamber, wherein a drive pinion of a drive shaft of the rotary piston pump is coupled with one of the rotor gears, wherein the rotor gears (12, 22) and the drive pinion are accommodated in the gear housing part, characterised in that the motor shaft of the drive motor comprises a cutout at the front end, in which the drive shaft of the rotary piston pump is received and coupled with the drive shaft, and that the drive motor is connected via a connection flange with a U-shaped cross-section to the housing, wherein the connection flange forms the rear closure cover of the housing and at the same time the front closure cover of the motor housing, wherein the connection flange comprises two flanges running in parallel and spaced apart from one another, which form a surrounding "U" opened outwards.