A casing comprising: an input-side casing part that houses at least an input shaft; and an output-side casing part that houses at least an output shaft, characterized in that the input-side casing part has an input-side bottom surface (P3a) and an input-side top surface (P2a), and the output-side casing part has an output-side bottom surface (P3b) and an output-side top surface (P2b), wherein a flat stepped portion having a height (H(P2a-P2b)) is formed on the top surface of the casing between the input-side top surface (P2a) of the input-side casing part and the output-side top surface (P2b) of the output-side casing part 10B or wherein a bottom-surface stepped portion having a height (H(P3a-P3b)) is formed on the bottom surface of the casing between the input-side bottom surface (P3a) of the input-side casing part and the output-side bottom surface (P3b) of the output-side casing part, the dimension (Y10A) between the input-side bottom surface (P3a) and the input-side top surface (P2a) of the input-side casing part in the axial direction of the output shaft is smaller than the dimension (Y10B) between the output-side bottom surface (P3b) and the output-side top surface (P2b) of the output-side casing part in the axial direction of the output shaft, the output-side bottom surface (P3b) or the output-side top surface (P2b) serves as an installation surface when the casing is installed so that the output shaft is parallel to a vertical direction, the output-side bottom surface (P3b) or the output-side top surface (P2b) includes mounting portions (34, 36) for vertical installation near four corners thereof, the input-side casing part includes an input-side horizontal installation surface (P6a) that serves as an installation surface when the casing is installed so that the output shaft is parallel to a horizontal direction, the output-side casing part includes an output-side horizontal installation surface (P6b) that serves as an installation surface when the casing is installed so that the output shaft is parallel to the horizontal direction, mounting portions for horizontal installation are provided near four corners of the output-side horizontal installation surface (P6b) of the output-side casing part and on the input-side horizontal installation surface (P6a) of the input-side casing part. an input-side casing part that houses at least an input shaft; and an output-side casing part that houses at least an output shaft, characterized in that the input-side casing part has an input-side bottom surface (P3a) and an input-side top surface (P2a), and the output-side casing part has an output-side bottom surface (P3b) and an output-side top surface (P2b), wherein a flat stepped portion having a height (H(P2a-P2b)) is formed on the top surface of the casing between the input-side top surface (P2a) of the input-side casing part and the output-side top surface (P2b) of the output-side casing part 10B or wherein a bottom-surface stepped portion having a height (H(P3a-P3b)) is formed on the bottom surface of the casing between the input-side bottom surface (P3a) of the input-side casing part and the output-side bottom surface (P3b) of the output-side casing part, the dimension (Y10A) between the input-side bottom surface (P3a) and the input-side top surface (P2a) of the input-side casing part in the axial direction of the output shaft is smaller than the dimension (Y10B) between the output-side bottom surface (P3b) and the output-side top surface (P2b) of the output-side casing part in the axial direction of the output shaft, the output-side bottom surface (P3b) or the output-side top surface (P2b) serves as an installation surface when the casing is installed so that the output shaft is parallel to a vertical direction, the output-side bottom surface (P3b) or the output-side top surface (P2b) includes mounting portions (34, 36) for vertical installation near four corners thereof, the input-side casing part includes an input-side horizontal installation surface (P6a) that serves as an installation surface when the casing is installed so that the output shaft is parallel to a horizontal direction, the output-side casing part includes an output-side horizontal installation surface (P6b) that serves as an installation surface when the casing is installed so that the output shaft is parallel to the horizontal direction, mounting portions for horizontal installation are provided near four corners of the output-side horizontal installation surface (P6b) of the output-side casing part and on the input-side horizontal installation surface (P6a) of the input-side casing part.