A head includes a head body, an additional member in which a contour shape obtained by a projection onto a plane has a rotational symmetric axis, and a holding mechanism for holding the additional member. The head body has a recess portion in which at least a part of the additional member can be fitted. The holding mechanism is constituted to enable a mutual transition of a holding state in which the additional member is held in the recess portion and a releasing state in which the hold is released. The recess portion inhibits a rotation around the rotational symmetric axis of the additional member in the holding state. The additional member is rotatable around the rotational symmetric axis in the releasing state. A phase of the additional member in the holding state is changed so that a position of a center of gravity of the head can be varied.