A turbomachine comprising: a rotating shaft; an impeller that is fixed to the rotating shaft and that compresses or expands a working fluid by rotating around an axial line of the rotating shaft; a bearing that rotatably supports the rotating shaft; and a casing that is arranged around the impeller in a circumferential direction of the rotating shaft and that includes a volute therein, the volute being a fluid passage for the working fluid compressed or expanded by rotation of the impeller, the volute having a cross-sectional area increasing in a flow direction of the working fluid when the turbomachine is compressing the working fluid, wherein: the casing has an outer wall that defines the volute, the casing includes a plurality of ribs that includes a first rib, and that is arranged on an outer peripheral surface of the outer wall, the outer peripheral surface of the outer wall includes a diagonal area and a tongue portion area, the first rib is arranged in the diagonal area, when seen from an axial direction of the rotating shaft, the tongue portion area includes a tongue portion that is in contact with a most upstream portion and a downstream portion of the volute, when seen from the axial direction of the rotating shaft, a center of the volute is located between the diagonal area and the tongue portion area, characterized in that the plurality of ribs is arranged only in the diagonal area, or the ribs of the plurality of ribs are spaced apart from each other in a winding direction of the volute and each extend over the volute, a degree of reinforcement of the diagonal area by the plurality of ribs being greater than a degree of reinforcement of the tongue portion area by the plurality of ribs, and the first rib has a largest height among the plurality of ribs, or the first rib has a dimension largest in the winding direction of the volute among the plurality of ribs. a rotating shaft; an impeller that is fixed to the rotating shaft and that compresses or expands a working fluid by rotating around an axial line of the rotating shaft; a bearing that rotatably supports the rotating shaft; and a casing that is arranged around the impeller in a circumferential direction of the rotating shaft and that includes a volute therein, the volute being a fluid passage for the working fluid compressed or expanded by rotation of the impeller, the volute having a cross-sectional area increasing in a flow direction of the working fluid when the turbomachine is compressing the working fluid, wherein: the casing has an outer wall that defines the volute, the casing includes a plurality of ribs that includes a first rib, and that is arranged on an outer peripheral surface of the outer wall, the outer peripheral surface of the outer wall includes a diagonal area and a tongue portion area, the first rib is arranged in the diagonal area, when seen from an axial direction of the rotating shaft, the tongue portion area includes a tongue portion that is in contact with a most upstream portion and a downstream portion of the volute, when seen from the axial direction of the rotating shaft, a center of the volute is located between the diagonal area and the tongue portion area, characterized in that the plurality of ribs is arranged only in the diagonal area, or the ribs of the plurality of ribs are spaced apart from each other in a winding direction of the volute and each extend over the volute, a degree of reinforcement of the diagonal area by the plurality of ribs being greater than a degree of reinforcement of the tongue portion area by the plurality of ribs, and the first rib has a largest height among the plurality of ribs, or the first rib has a dimension largest in the winding direction of the volute among the plurality of ribs. the casing has an outer wall that defines the volute, the casing includes a plurality of ribs that includes a first rib, and that is arranged on an outer peripheral surface of the outer wall, the outer peripheral surface of the outer wall includes a diagonal area and a tongue portion area, the first rib is arranged in the diagonal area, when seen from an axial direction of the rotating shaft, the tongue portion area includes a tongue portion that is in contact with a most upstream portion and a downstream portion of the volute, when seen from the axial direction of the rotating shaft, a center of the volute is located between the diagonal area and the tongue portion area, characterized in that the plurality of ribs is arranged only in the diagonal area, or the ribs of the plurality of ribs are spaced apart from each other in a winding direction of the volute and each extend over the volute, a degree of reinforcement of the diagonal area by the plurality of ribs being greater than a degree of reinforcement of the tongue portion area by the plurality of ribs, and the plurality of ribs is arranged only in the diagonal area, or the ribs of the plurality of ribs are spaced apart from each other in a winding direction of the volute and each extend over the volute, a degree of reinforcement of the diagonal area by the plurality of ribs being greater than a degree of reinforcement of the tongue portion area by the plurality of ribs, and the first rib has a largest height among the plurality of ribs, or the first rib has a dimension largest in the winding direction of the volute among the plurality of ribs.