An assembling apparatus that assembles a retainer by engaging a pair of cotter pieces with an upper end of a valve stem, characterized by comprising: a first assembling head configured to place the retainer on a valve spring provided to surround the upper end of the valve stem and assemble the retainer; a first moving unit (52a, 53a, 54a, 55a) configured to move the first assembling head between a supply position of the retainer and an assembling position; a second assembling head configured to hold the pair of cotter pieces and engages the pair of cotter pieces with the upper end of the valve stem; and a second moving unit (52b, 53b, 54b, 55b) configured to move the second assembling head between a supply position of the pair of cotter pieces and the valve stem, wherein the first assembling head comprises: a holding unit configured to hold the retainer at a lower end; and a push unit configured to push out the retainer held by the holding unit, the first moving unit (5, 52a, 53a, 54a, 55a) comprises: a horizontal moving unit (52a, 53a, 54a) configured to move the first assembling head in a horizontal direction; and an elevating unit configured to raise/lower the first assembling head, and the push unit comprises: a push member supported to be able to advance/retreat along an elevating direction of the first assembling head by the elevating unit; and biasing means for biasing the push member in an advancing direction. a first assembling head configured to place the retainer on a valve spring provided to surround the upper end of the valve stem and assemble the retainer; a first moving unit (52a, 53a, 54a, 55a) configured to move the first assembling head between a supply position of the retainer and an assembling position; a second assembling head configured to hold the pair of cotter pieces and engages the pair of cotter pieces with the upper end of the valve stem; and a second moving unit (52b, 53b, 54b, 55b) configured to move the second assembling head between a supply position of the pair of cotter pieces and the valve stem, wherein the first assembling head comprises: a holding unit configured to hold the retainer at a lower end; and a push unit configured to push out the retainer held by the holding unit, a holding unit configured to hold the retainer at a lower end; and a push unit configured to push out the retainer held by the holding unit, the first moving unit (5, 52a, 53a, 54a, 55a) comprises: a horizontal moving unit (52a, 53a, 54a) configured to move the first assembling head in a horizontal direction; and an elevating unit configured to raise/lower the first assembling head, and a horizontal moving unit (52a, 53a, 54a) configured to move the first assembling head in a horizontal direction; and an elevating unit configured to raise/lower the first assembling head, and the push unit comprises: a push member supported to be able to advance/retreat along an elevating direction of the first assembling head by the elevating unit; and biasing means for biasing the push member in an advancing direction. a push member supported to be able to advance/retreat along an elevating direction of the first assembling head by the elevating unit; and biasing means for biasing the push member in an advancing direction.