The invention relates to a liquid filter, having a filter housing, which contains a receiving space for a ring filter element. The liquid filter further has a no-load duct, which is opened when the ring filter element is taken out. A pin is formed on a lower end disc of the ring filter element, the pin projects parallel to the longitudinal axis. A ramp is formed on the bottom of the receiving space. The ramp is matched to the pin in such a manner that, when the ring filter element is inserted into the filter housing, the pin slides downwards on the ramp and penetrates the opening of the no-load duct. A structure is in the receiving space, the structure allows functional insertion of the ring filter element if the ring filter element slides with its pin along the ramp until the pin penetrates the opening of the no-load duct.