To provide a rail mounting structure which allows execution of works so that no steps are generated in the floor and which does not includes any place hard to execute necessary works.

A threshold member having approx. the same thickness as a floor finishing member 2 is attached to an appropriate place on the floor, and the oversurface of the floor finishing member 2 is made flush substantially with the threshold member oversurface. A rail mounting groove 4 is furnished at the threshold oversurface, and a rail member 5 is installed at the threshold oversurface so as to be accommodated in the groove 4. Three grooves 11 are furnished at the oversurface of the rail member 5, among which the two on the sides 11 are used as fixing grooves 11a for driving a fixture 12 from them so that the rail member 5 is fixed to the threshold member, while the central groove 11 is used as a running groove 11b for supporting door rollers 7 with possibility of traveling freely.