A head fixing mechanism includes a fixture fixed to a trunk portion and a coupling member which is threadably engaged with a fixture. The coupling member includes a main body which has a head portion that presses a head and a shaft portion that is threadably engaged with the fixture, and in which an axially extending insertion hole is formed, and an insertion member which is threadably engaged with the main body within the insertion hole. A bottom of the insertion hole is formed in the shaft portion, and in the shaft portion, a slit having a width smaller than the inside diameter of the insertion hole is formed, which divides the shaft portion in a circumferential direction. The insertion member is formed in a tapered shape, and the outside diameter of the leading end thereof is formed to be smaller than the width of the slit.