To provide a traveling vehicle system for enhancing the conveying efficiency of an unmanned traveling vehicle.

The traveling vehicle system includes a substrate treatment device for storing a substrate or performing predetermined treatment, the unmanned traveling vehicle for conveying the substrate along a track extending toward the substrate treatment device in a state that the substrate is housed in a substrate cassette 1, and a step number changing device MT having a first substrate cassette holding portion K1 for holding the substrate cassette 1 conveyed from the unmanned traveling vehicle, a second substrate cassette holding portion K2 for holding the substrate cassette 1 above the first substrate cassette holding portion K1, and lifting portions 16a and 16b for vertically moving the substrate cassette 1 between the first substrate cassette holding portion K1 and the second substrate cassette holding portion K2.