A cutting insert, comprising: an upper surface; a lower surface; a side surface which is connected to the upper surface and the lower surface, and comprises a first side surface and a second side surface in order; and a cutting edge comprising: a first flat cutting edge and a first major cutting edge in order in an intersection region of the first side surface and the upper surface, and a second flat cutting edge and a second major cutting edge in order in an intersection region of the second side surface and the upper surface, wherein the first side surface comprises a first chamfered side surface with a curved shape, a first corner side surface with a planar shape, and a first major side surface in order, the second side surface comprises a second chamfered side surface with a curved shape, a second corner side surface with a planar shape, and a second major side surface in order, the intersection region of the second side surface and the upper surface comprises a first intersection region of the second chamfered side surface and the upper surface, and a second intersection region of the second corner side surface and the upper surface, the second flat cutting edge which is located from the first intersection region to the second intersection region, and has a lowermost portion in the first intersection region in a side view, characterized in that the second flat cutting edge has a concave curve with respect to a reference plane perpendicular to a central axis of the cutting insert in a side view. an upper surface; a lower surface; a side surface which is connected to the upper surface and the lower surface, and comprises a first side surface and a second side surface in order; and a cutting edge comprising: a first flat cutting edge and a first major cutting edge in order in an intersection region of the first side surface and the upper surface, and a second flat cutting edge and a second major cutting edge in order in an intersection region of the second side surface and the upper surface, wherein a first flat cutting edge and a first major cutting edge in order in an intersection region of the first side surface and the upper surface, and a second flat cutting edge and a second major cutting edge in order in an intersection region of the second side surface and the upper surface, wherein the first side surface comprises a first chamfered side surface with a curved shape, a first corner side surface with a planar shape, and a first major side surface in order, the second side surface comprises a second chamfered side surface with a curved shape, a second corner side surface with a planar shape, and a second major side surface in order, the intersection region of the second side surface and the upper surface comprises a first intersection region of the second chamfered side surface and the upper surface, and a second intersection region of the second corner side surface and the upper surface, a first intersection region of the second chamfered side surface and the upper surface, and a second intersection region of the second corner side surface and the upper surface, the second flat cutting edge which is located from the first intersection region to the second intersection region, and has a lowermost portion in the first intersection region in a side view, characterized in that the second flat cutting edge has a concave curve with respect to a reference plane perpendicular to a central axis of the cutting insert in a side view.