To provide an expansion joint device for a floor capable of absorbing relative displacement in an approaching/separating direction of buildings due to an earthquake and relative displacement in a direction orthogonal on the same plane to the approaching/separating direction and having a floor face having no level difference and flush with floor faces on the two adjacent building side.

The expansion joint device for the floor comprises a fixed floor body 21 mounted to a facing part 34a through a support plate 23 and a first support means 24 so that its upper end is flush with the floor face 36a on the building 29a side, and a movable floor body 22 mounted to a facing part 34b through a hinge member 26 with a horizontal axis C and a second support means 27 so that its tip part is slidably supported on the support plate 23 and that its upper end is flush with the floor face 36b on the building 29b side which is flush with the floor face 36a. Tip side comb tooth parts 25A, 25B of both floor bodies 21, 22 are inserted differently from opposite directions from each other to absorb the relative displacement in the approaching/separating direction of the buildings 29a, 29b and to absorb the relative displacement in the direction orthogonal on the same plane to the approaching/separating direction by the first and second support means 24, 27.