A high-frequency plasma spark plug which includes: an insulator which has an axial hole extending in the axis direction; a center electrode which is inserted into a distal end side of the axial hole; a terminal electrode which is inserted into a rear end side of the axial hole, and is electrically connected to the center electrode; a cylindrical main fitting which is mounted on an outer periphery of the insulator, characterized in that it further includes a coaxial cable which has an inner conductor and a cylindrical outer conductor arranged on an outer periphery of the inner conductor and may be a cable in which a metal-made pipe is arranged on an outer periphery of an inner conductor, the inner conductor is connected to the terminal electrode and the outer conductor is connected to the main fitting, and high frequency power generated by a predetermined high-frequency power source is supplied via the coaxial cable thus generating high frequency plasma, wherein the main fitting comprises: a large diameter portion which bulges radially outward; and a connection portion which is brought into contact with the outer conductor, and the connection portion is formed closer to a rear end side in the axis direction than the large diameter portion, an outer periphery of the connection portion has a cylindrical shape which extends along the axis direction, and a length of the connection portion along the axis is set to not less than 0.5mm and not more than 5mm. an insulator which has an axial hole extending in the axis direction; a center electrode which is inserted into a distal end side of the axial hole; a terminal electrode which is inserted into a rear end side of the axial hole, and is electrically connected to the center electrode; a cylindrical main fitting which is mounted on an outer periphery of the insulator, a coaxial cable which has an inner conductor and a cylindrical outer conductor arranged on an outer periphery of the inner conductor and may be a cable in which a metal-made pipe is arranged on an outer periphery of an inner conductor, the inner conductor is connected to the terminal electrode and the outer conductor is connected to the main fitting, and high frequency power generated by a predetermined high-frequency power source is supplied via the coaxial cable thus generating high frequency plasma, wherein the main fitting comprises: a large diameter portion which bulges radially outward; and a connection portion which is brought into contact with the outer conductor, and the connection portion is formed closer to a rear end side in the axis direction than the large diameter portion, an outer periphery of the connection portion has a cylindrical shape which extends along the axis direction, and a length of the connection portion along the axis is set to not less than 0.5mm and not more than 5mm. a large diameter portion which bulges radially outward; and a connection portion which is brought into contact with the outer conductor, and the connection portion is formed closer to a rear end side in the axis direction than the large diameter portion, an outer periphery of the connection portion has a cylindrical shape which extends along the axis direction, and a length of the connection portion along the axis is set to not less than 0.5mm and not more than 5mm.