Heretofore, it has been common practice to use small-diameter work rolls to roll hard materials, such as a magnetic steel strip, a stainless steel strip, and a high-tension steel strip, with a high strip thickness gauge accuracy. Rolling mills using such small-diameter work rolls are configured such that horizontally split housings, namely, a top inner housing and a bottom inner housing are used to respectively support a top roll group, which supports a top work roll and includes rolls arranged in a clustered form, and a bottom roll group, which supports a bottom work roll and includes rolls arranged in a clustered form. Further, a drive-side outer housing and a work-side outer housing are used to support the top inner housing and the bottom inner housing.
A cluster-type multistage rolling mill of such type is disclosed in Patent Literature 1, for example.