The sections of ferrite coupled lines with longitudinal magnetization (FCL) play important role in novel class of integrated nonreciprocal devices. The circulator is designed for instance as a cascade of a section of FCL and T-hybrid. In this paper a slot line FCL is developed and applied in design of the circulator. The simulated behavior of the FCL indicates the Faraday's rotation phenomenon performing a periodical exchange of em wave energy between the coupled lines of the structure. It also predicts that a section of FCL with pi/4 rotation is useful for circulator design. The FCL circulator was designed and produced in finline technology. The measured parameters of the circulator agree well with the ones theoretically predicted.