Millable plugging device for use in a well pipe, comprising: - a inner mandrel comprising a first mandrel section and a second mandrel section, wherein the first mandrel section has a cylindrical outer surface and wherein the second mandrel section has a cylindrical outer surface; - an outer housing provided radially outside at least parts of the inner mandrel, wherein the outer housing comprises a first housing section and a second housing section, wherein the second housing section comprises a cylindrical compartment; - a sealing assembly provided on the cylindrical outer surface of the second mandrel section, wherein the sealing assembly is configured to be brought from a run state to a set state by a relative axial displacement between the inner mandrel and the outer housing, wherein, in the set state, the sealing assembly is brought in contact with the inner surface of the well pipe; wherein the millable plugging device has a first center axis; characterized in that: at least a part of the second mandrel section is provided within the compartment in the set state; the cylindrical outer surface of the second mandrel section has a second center axis (I2b) which is radially offset from and parallel to the first center axis; the axis of the cylindrical compartment of the second housing section coincides with said second center axis (I2b); the axis of the first mandrel section and the axis of the first housing section coincide with the first center axis; a cylindrical inner surface of the first housing section defines an axial opening through which the first mandrel section is guided; the inner diameter of the cylindrical compartment, the inner diameter of the axial opening and the outer diameters of the cylindrical outer surfaces of the first and second mandrel sections (2a, 2b) are adapted to prevent rotation of the mandrel within the housing in the set state; rotation of the inner mandrel relative to the sealing assembly in the set state is bringing a third mandrel section of the mandrel to a locked state, in which further rotation of the inner mandrel is prevented; and while being brought to the locked state, the mandrel is also being rotated about the second center axis (I2b) until a first contact area of the third mandrel section is brought into contact with the well pipe. - a inner mandrel comprising a first mandrel section and a second mandrel section, wherein the first mandrel section has a cylindrical outer surface and wherein the second mandrel section has a cylindrical outer surface; - an outer housing provided radially outside at least parts of the inner mandrel, wherein the outer housing comprises a first housing section and a second housing section, wherein the second housing section comprises a cylindrical compartment; - a sealing assembly provided on the cylindrical outer surface of the second mandrel section, wherein the sealing assembly is configured to be brought from a run state to a set state by a relative axial displacement between the inner mandrel and the outer housing, wherein, in the set state, the sealing assembly is brought in contact with the inner surface of the well pipe; at least a part of the second mandrel section is provided within the compartment in the set state; the cylindrical outer surface of the second mandrel section has a second center axis (I2b) which is radially offset from and parallel to the first center axis; the axis of the cylindrical compartment of the second housing section coincides with said second center axis (I2b); the axis of the first mandrel section and the axis of the first housing section coincide with the first center axis; a cylindrical inner surface of the first housing section defines an axial opening through which the first mandrel section is guided; the inner diameter of the cylindrical compartment, the inner diameter of the axial opening and the outer diameters of the cylindrical outer surfaces of the first and second mandrel sections (2a, 2b) are adapted to prevent rotation of the mandrel within the housing in the set state; rotation of the inner mandrel relative to the sealing assembly in the set state is bringing a third mandrel section of the mandrel to a locked state, in which further rotation of the inner mandrel is prevented; and while being brought to the locked state, the mandrel is also being rotated about the second center axis (I2b) until a first contact area of the third mandrel section is brought into contact with the well pipe.