Provided is a liquid level detection device with excellent bonding strength between components. This liquid level detection device is provided with a holder which has a magnet inside and which rotates in response to displacement of a float floating in the liquid the level of which is to be measured, a main body unit which rotatably supports the holder and has a magnetic detection element which detects magnetic pole change accompanying rotational movement of the magnet, and a cover which covers the holder and which is attached by welding to the main body unit to prevent the holder from falling away from the main body unit. The cover is welded to the main body unit by melting protrusions formed on the main body unit.