Provided is a liquid-surface detection device in which a holder, which turns concurrently with movement of a float arm that moves based on a change in a liquid surface, is securely and rotatably mounted on a frame. A liquid-surface detection device includes a holder that includes an upper piece and a lower piece, and a frame that includes a bearing, a first groove, and a second groove. When mounting the holder onto the frame, the first groove guides the lower piece to the second groove, and the second groove further guides the lower piece to the bearing part so that the bearing part of the frame is disposed between the upper piece and the lower piece of the holder.