Milling tool and workpiece machining method

A milling tool comprising a shank section and a head which is provided on a tip end side of the shank section and which has a cutting edge, wherein

• the head has an expanding diameter section which gradually increases in diameter from a base end portion adjoining the shank section in a tip end direction, and a contracting diameter section which gradually decreases in diameter from a maximum diameter section in the tip end direction, and
• the milling tool comprises:
• at least one cutting edge provided on the expanding diameter section and at least one cutting edge provided on the contracting diameter section, and
• at least one tip end cutting edge which extends from the contracting diameter section to a center axis of the milling tool in a tip end section of the head,
• the cutting edge of the expanding diameter section is formed on an upper insert attached to the expanding diameter section and the cutting edge of the contracting diameter section is formed on a lower insert attached to the contracting diameter section,
• each of the upper insert and the lower insert has a first cutting edge linearly extending from one end thereof, and
• a tip end insert having a linear main cutting edge along a conical surface defined by a path of the first cutting edge of the lower insert when the milling tool rotates.