An ignition coil for an internal-combustion engine comprising: a primary coil through which a primary current is made to flow; a secondary coil that generates a secondary voltage by intersecting a first magnetic flux generated by the primary coil; a center core inserted into a center hole of the primary coil and a center hole of the secondary coil; an annular side core that surrounds the primary coil and the secondary coil, the annular side core being joined to the center core and forming a magnetic circuit through which the first magnetic flux permeates; and a permanent magnet that is disposed between the center core and the side core, the permanent magnet emitting a second magnetic flux to the magnetic circuit in a direction opposite to that of the first magnetic flux and applying a magnetic bias; wherein the side core includes a protruded portion that protrudes towards a lateral side of an end portion of the center core joined to the permanent magnet, wherein a gap is provided between the lateral side of the end portion of the center core and the protruded portion of the side core, wherein the gap is provided so that the magnetic reluctance acting on the first magnetic flux is smaller when the gap serves as a magnetic path than when the permanent magnet serves as a magnetic path of the first magnetic flux, whereas the magnetic reluctance of the gap is larger than the magnetic reluctance of the magnetic circuit that connects the poles of the permanent magnet through the side core without permeating the gap, wherein the center core is formed in a T-shape that includes a vertical portion that is inserted into the center hole of the primary coil and the center hole of the secondary coil, and a horizontal portion in which the end portion joined to the permanent magnet is provided so as to extend in a perpendicular direction with respect to the vertical portion, and wherein the gap is formed by opposing an end portion of the horizontal portion and the protruded portion of the side core. a primary coil through which a primary current is made to flow; a secondary coil that generates a secondary voltage by intersecting a first magnetic flux generated by the primary coil; a center core inserted into a center hole of the primary coil and a center hole of the secondary coil; an annular side core that surrounds the primary coil and the secondary coil, the annular side core being joined to the center core and forming a magnetic circuit through which the first magnetic flux permeates; and a permanent magnet that is disposed between the center core and the side core, the permanent magnet emitting a second magnetic flux to the magnetic circuit in a direction opposite to that of the first magnetic flux and applying a magnetic bias; wherein the side core includes a protruded portion that protrudes towards a lateral side of an end portion of the center core joined to the permanent magnet, wherein a gap is provided between the lateral side of the end portion of the center core and the protruded portion of the side core, wherein the gap is provided so that the magnetic reluctance acting on the first magnetic flux is smaller when the gap serves as a magnetic path than when the permanent magnet serves as a magnetic path of the first magnetic flux, whereas the magnetic reluctance of the gap is larger than the magnetic reluctance of the magnetic circuit that connects the poles of the permanent magnet through the side core without permeating the gap, wherein the center core is formed in a T-shape that includes a vertical portion that is inserted into the center hole of the primary coil and the center hole of the secondary coil, and a horizontal portion in which the end portion joined to the permanent magnet is provided so as to extend in a perpendicular direction with respect to the vertical portion, and wherein the gap is formed by opposing an end portion of the horizontal portion and the protruded portion of the side core.