To provide a fitting structure of a stair and a unit building using the structure which does not impair the appearance when going up and down the stair.

An upstairs building unit and a downstairs building unit are vertically connected to construct the unit building. In this fitting structure of the stair constituted of a building unit body and a plurality of treadboard units 11, 21, the treadboard unit 1 is composed of treadboard 11 and a decoration side plate 12 erected at the upper side of the longitudinal end of the tread-board 11, and the treadboard 21 of the lowermost treadboard unit 2 of the upstairs building unit is provided with notches in which the decoration side plate 22 of the treadboard unit 2 and the decoration side plate 12 of the treadboard unit 1 right under the treadboard unit 2 are inserted respectively.