The invention relates to an alarm device, and aims to provide an automatic alarm device for a fuel filter of an internal combustion engine. The alarm device is provided with a sensor and comprises an unfiltered fuel cavity shell provided with an unfiltered fuel inlet and a filtered fuel cavity shell provided with a filtered fuel inlet, wherein the two shells are butted to form a cavity, a piston is arranged in the cavity, and two springs are arranged on two sides of the piston and between inner walls at two ends of the cavity respectively; the unfiltered fuel cavity shell and the filtered fuel cavity shell are provided with an adjusting conductive screw penetrating through the shell respectively, the piston is penetrated by a conductive rod, and two ends of the conductive rod correspond to the ends of the two adjusting conductive screws respectively; and the conductive rod is connected with a conducting wire for connecting with one electrode of a power supply, and the two adjusting conductive screws are connected with the conducting wire respectively for connecting with the other electrode of the power supply. The alarm device not only has the function of blockage alarm, but also has the function of breakdown alarm. The alarm device has the advantages of simple structure, small volume, flexible arrangement, low manufacturing cost and easy popularization.