An assembling head configured to assemble a retainer by engaging a pair of cotter pieces with an upper end of a valve stem, comprising: a storage member including a storage chamber configured to store the pair of cotter pieces from below in a separated state with a gap capable of receiving the upper end of the valve stem; and a holding unit configured to hold, in the separated state, the pair of cotter pieces stored in the storage chamber and cancel the separated state, wherein the holding unit is configured to press the pair of cotter pieces against the upper end of the valve stem, wherein the storage member includes: a first abutting portion provided on an inner wall of the storage chamber, against which upper end faces of the pair of cotter pieces stored in the storage chamber abut; and a second abutting portion provided on an upper wall of the storage chamber, against which an upper end face of the valve stem inserted from below the storage chamber abuts, the second abutting portion positions the valve stem relative to the storage chamber in an axial direction of the valve stem, the first abutting portion positions the pair of cotter pieces relative to the storage chamber in the axial direction, characterized in that the holding unit comprises: a separating member provided to be insertable to or removable from a center of the storage chamber from above the storage member and including spacer portions intervening between the pair of cotter pieces; a driving unit configured to move the separating member in an insertion and removal direction relative to the storage member; and a pair of pressing members provided to be able to advance to or retreat from the storage chamber from both sides of the storage member and configured to bias the pair of cotter pieces toward the center of the storage chamber. a storage member including a storage chamber configured to store the pair of cotter pieces from below in a separated state with a gap capable of receiving the upper end of the valve stem; and a holding unit configured to hold, in the separated state, the pair of cotter pieces stored in the storage chamber and cancel the separated state, wherein the holding unit is configured to press the pair of cotter pieces against the upper end of the valve stem, wherein the storage member includes: a first abutting portion provided on an inner wall of the storage chamber, against which upper end faces of the pair of cotter pieces stored in the storage chamber abut; and a second abutting portion provided on an upper wall of the storage chamber, against which an upper end face of the valve stem inserted from below the storage chamber abuts, the second abutting portion positions the valve stem relative to the storage chamber in an axial direction of the valve stem, the first abutting portion positions the pair of cotter pieces relative to the storage chamber in the axial direction, characterized in that a first abutting portion provided on an inner wall of the storage chamber, against which upper end faces of the pair of cotter pieces stored in the storage chamber abut; and a second abutting portion provided on an upper wall of the storage chamber, against which an upper end face of the valve stem inserted from below the storage chamber abuts, the second abutting portion positions the valve stem relative to the storage chamber in an axial direction of the valve stem, the first abutting portion positions the pair of cotter pieces relative to the storage chamber in the axial direction, characterized in that a separating member provided to be insertable to or removable from a center of the storage chamber from above the storage member and including spacer portions intervening between the pair of cotter pieces; a driving unit configured to move the separating member in an insertion and removal direction relative to the storage member; and a pair of pressing members provided to be able to advance to or retreat from the storage chamber from both sides of the storage member and configured to bias the pair of cotter pieces toward the center of the storage chamber.