A rotary electric machine including: a protective cover; a casing formed of a front bracket and a rear bracket; and a component, wherein the component is arranged outside the casing on a rear side, wherein the protective cover is mounted to the rear bracket and configured to, protect the component, the protective cover comprising: a bottom portion formed into an approximately circular shape; a side wall, which extends from the bottom portion along an outer periphery of the bottom portion; and an opening portion, which has an approximately circular shape and is formed by an end of the side wall, which is located on a side opposite to the bottom portion, characterized in that the side wall has a plurality of guide ribs (31,32), which are formed on an inner side thereof and are arranged apart from each other along the outer periphery of the bottom portion, wherein each of the plurality of guide ribs (31,32) extends from the bottom portion toward the opening portion and has a chamfered portion at an end on a side closer to the opening portion, wherein each of the plurality of guide ribs (31,32) is formed so that a height of a top portion of each of the plurality of guide ribs (31,32) from the side wall allows the top portion to be brought into contact with a side surface of the component when the protective cover is mounted to the rotary electric machine, wherein the component comprises a rectifying device, and wherein at least one of the plurality of guide ribs (31,32) is brought into contact with a side surface of the rectifying device when the protective cover is mounted to the rotary electric machine. a protective cover; a casing formed of a front bracket and a rear bracket; and a component, wherein the component is arranged outside the casing on a rear side, wherein the protective cover is mounted to the rear bracket and configured to, protect the component, the protective cover comprising: a bottom portion formed into an approximately circular shape; a side wall, which extends from the bottom portion along an outer periphery of the bottom portion; and an opening portion, which has an approximately circular shape and is formed by an end of the side wall, which is located on a side opposite to the bottom portion, characterized in that the side wall has a plurality of guide ribs (31,32), which are formed on an inner side thereof and are arranged apart from each other along the outer periphery of the bottom portion, wherein each of the plurality of guide ribs (31,32) extends from the bottom portion toward the opening portion and has a chamfered portion at an end on a side closer to the opening portion, wherein each of the plurality of guide ribs (31,32) is formed so that a height of a top portion of each of the plurality of guide ribs (31,32) from the side wall allows the top portion to be brought into contact with a side surface of the component when the protective cover is mounted to the rotary electric machine, wherein the component comprises a rectifying device, and wherein at least one of the plurality of guide ribs (31,32) is brought into contact with a side surface of the rectifying device when the protective cover is mounted to the rotary electric machine. a bottom portion formed into an approximately circular shape; a side wall, which extends from the bottom portion along an outer periphery of the bottom portion; and an opening portion, which has an approximately circular shape and is formed by an end of the side wall, which is located on a side opposite to the bottom portion, characterized in that the side wall has a plurality of guide ribs (31,32), which are formed on an inner side thereof and are arranged apart from each other along the outer periphery of the bottom portion, wherein each of the plurality of guide ribs (31,32) extends from the bottom portion toward the opening portion and has a chamfered portion at an end on a side closer to the opening portion, wherein each of the plurality of guide ribs (31,32) is formed so that a height of a top portion of each of the plurality of guide ribs (31,32) from the side wall allows the top portion to be brought into contact with a side surface of the component when the protective cover is mounted to the rotary electric machine, wherein the component comprises a rectifying device, and wherein at least one of the plurality of guide ribs (31,32) is brought into contact with a side surface of the rectifying device when the protective cover is mounted to the rotary electric machine.