A method and apparatus for applying a magnetic liquid to a moving web by using an extruder head for ejecting the magnetic liquid onto the web. A by-pass line for returning the liquid to a feed source through a changeover valve is provided immediately upstream to the extruder head. The liquid is supplied to the by-pass line through the valve after the extruder head is filled with the liquid before the first starting of the application or the resumption thereof. The position of the valve is shifted at the time of the first starting of the application or the resumption thereof so as to supply the liquid to the head to perform the application.