An assembling apparatus that places a retainer on a valve spring provided to surround an upper end of a valve stem, comprising: an assembling head configured to place the retainer on the valve spring; and a moving unit (52a, 53a, 54a, 55a) configured to move the assembling head between a supply position of the retainer and an assembling position, characterized in that the assembling head comprises: a holding unit configured to hold the retainer separated from a pair of cotter pieces at a lower end; and a push unit configured to push out the retainer toward the valve spring downward with engaging with the retainer when holding of the retainer by the holding unit is canceled, the moving unit (52a, 53a, 54a, 55a) comprises: a horizontal moving unit (52a, 53a, 54a) configured to move the assembling head in a horizontal direction; and an elevating unit configured to raise/lower the assembling head, and the push unit comprises: a push member supported to be able to advance/retreat along an elevating direction of the assembling head by the elevating unit; and biasing means for biasing the push member in an advancing direction, wherein the holding unit includes two holding members which face each other in a direction orthogonal to the elevating direction, and the push member is located in a middle between the two holding members in the direction orthogonal to the elevating direction. an assembling head configured to place the retainer on the valve spring; and a moving unit (52a, 53a, 54a, 55a) configured to move the assembling head between a supply position of the retainer and an assembling position, characterized in that the assembling head comprises: a holding unit configured to hold the retainer separated from a pair of cotter pieces at a lower end; and a push unit configured to push out the retainer toward the valve spring downward with engaging with the retainer when holding of the retainer by the holding unit is canceled, the moving unit (52a, 53a, 54a, 55a) comprises: a horizontal moving unit (52a, 53a, 54a) configured to move the assembling head in a horizontal direction; and an elevating unit configured to raise/lower the assembling head, and the push unit comprises: a push member supported to be able to advance/retreat along an elevating direction of the assembling head by the elevating unit; and biasing means for biasing the push member in an advancing direction, wherein the holding unit includes two holding members which face each other in a direction orthogonal to the elevating direction, and the push member is located in a middle between the two holding members in the direction orthogonal to the elevating direction. a holding unit configured to hold the retainer separated from a pair of cotter pieces at a lower end; and a push unit configured to push out the retainer toward the valve spring downward with engaging with the retainer when holding of the retainer by the holding unit is canceled, the moving unit (52a, 53a, 54a, 55a) comprises: a horizontal moving unit (52a, 53a, 54a) configured to move the assembling head in a horizontal direction; and an elevating unit configured to raise/lower the assembling head, and the push unit comprises: a push member supported to be able to advance/retreat along an elevating direction of the assembling head by the elevating unit; and biasing means for biasing the push member in an advancing direction, wherein the holding unit includes two holding members which face each other in a direction orthogonal to the elevating direction, and the push member is located in a middle between the two holding members in the direction orthogonal to the elevating direction. a horizontal moving unit (52a, 53a, 54a) configured to move the assembling head in a horizontal direction; and an elevating unit configured to raise/lower the assembling head, and the push unit comprises: a push member supported to be able to advance/retreat along an elevating direction of the assembling head by the elevating unit; and biasing means for biasing the push member in an advancing direction, wherein the holding unit includes two holding members which face each other in a direction orthogonal to the elevating direction, and the push member is located in a middle between the two holding members in the direction orthogonal to the elevating direction. a push member supported to be able to advance/retreat along an elevating direction of the assembling head by the elevating unit; and biasing means for biasing the push member in an advancing direction, wherein the holding unit includes two holding members which face each other in a direction orthogonal to the elevating direction, and the push member is located in a middle between the two holding members in the direction orthogonal to the elevating direction.