A door closer according to the invention comprises a filter (12) placed across the flow channel (9) to filter the fluid flowing to the control valve (11). The flow channel is within the door closer body and guides the flow of fluid from a first side of the piston system to a second side. The control valve is located in connection with the flow channel. The filter is arranged so that the incoming flow to the filter is allowed to pass through a filter area greater than the cross-sectional area of the flow channel (9), and the outgoing flow from the filter is allowed to pass through a filter area equal to the cross-sectional area of the flow channel.