A system for treating a spinal disease, said system comprising: an interbody spacer which is curved in a horizontal direction and used by being inserted between vertebral bodies, the interbody spacer including a pair of contact surfaces to be in contact with each of the vertebral bodies, and including a ventral surface which is a side surface on a ventral side having a convex shape in plan view and which connects said pair of contact surfaces on the ventral side, and a dorsal surface which is a side surface on a dorsal side having a concave shape in plan view and which connects said pair of contact surfaces on the dorsal side, the interbody spacer including an engagement portion which is located along the direction in which the interbody spacer is curved, in the ventral surface and/or the dorsal surface; and a guiding tool for guiding the interbody spacer to a predetermined position between the vertebral bodies, the guiding tool including, on a distal end side, a guide rail portion said guide rail portion having a radius of curvature in plan view which is the same as a radius of curvature of the ventral surface or the dorsal surface in plan view; characterized in that the guide rail portion is designed in such a way to be fitted into either one of the engagement portion in the ventral surface and the engagement portion in the dorsal surface, wherein either one of the transverse cross-section of the engagement portion and the transverse cross-section of the guide rail portion has a concave shape, while the other has a convex shape that corresponds to the concave shape, and the concave shape includes a portion having a maximum width that is greater than an opening width of the concave shape that is opposed to the convex shape. an interbody spacer which is curved in a horizontal direction and used by being inserted between vertebral bodies, the interbody spacer including a pair of contact surfaces to be in contact with each of the vertebral bodies, and including a ventral surface which is a side surface on a ventral side having a convex shape in plan view and which connects said pair of contact surfaces on the ventral side, and a dorsal surface which is a side surface on a dorsal side having a concave shape in plan view and which connects said pair of contact surfaces on the dorsal side, the interbody spacer including an engagement portion which is located along the direction in which the interbody spacer is curved, in the ventral surface and/or the dorsal surface; and a guiding tool for guiding the interbody spacer to a predetermined position between the vertebral bodies, the guiding tool including, on a distal end side, a guide rail portion said guide rail portion having a radius of curvature in plan view which is the same as a radius of curvature of the ventral surface or the dorsal surface in plan view; characterized in that the guide rail portion is designed in such a way to be fitted into either one of the engagement portion in the ventral surface and the engagement portion in the dorsal surface, wherein either one of the transverse cross-section of the engagement portion and the transverse cross-section of the guide rail portion has a concave shape, while the other has a convex shape that corresponds to the concave shape, and the concave shape includes a portion having a maximum width that is greater than an opening width of the concave shape that is opposed to the convex shape.