A vertical membrane module unit is allowed to be installed even at a place with a low ceiling. A housing containing a vertical membrane module unit is held by a housing holder. The housing holder holds the housing tiltably such that the housing can be positioned with the longitudinal direction of the membrane module unit being substantially vertical when treated water is extracted, and the housing can be positioned with the longitudinal direction of the membrane module unit being substantially horizontal when the membrane module unit is removed from the housing.