A new all-fibre device for controlling the state of polarisation (SOP) in a single-mode fibre is proposed. It consists of two fibre elements called rotatable fibre crank (RFC) elements, each of which is equivalent to a rotatable phase plate. It features a very low insertion loss when used in a fibre circuit, because it can be spliced with input and output fibres, and that the control is endless as in a rotatable phase plate. The effectiveness of the SOP control using this device is verified experimentally.