To provide an assembling device capable of accurately aligning and inserting even a large shrink-fit holder by sensitively laterally moving and inclining the holder by weak contact of the shank of a tool thereon.

A part set support part 6 comprises a body base 33, an air jet hole 34 for jetting air upward which is formed in the upper part of the body base 33, and a floating base 39 on which the shrink-fit holder 5 floated by the jet of the air from the air jet hole 34 which is placed on the body base 33 is so placed and set that the flange 38 thereof is brought into contact with the floating base. A part of the hip of the shank 11 of the tool 10 is lightly aligned with a part insertion hole 42 enlarged by heating the part holding part 41 of the shrink-fit holder 5 when the floating base 39 is floated by the jet of the air from the air jet hole 34.