Abstract: A locking device for a tank having a spout and a cap attached to the spout, the device including a main body having a first and second hinging portion. The first and second hinging portions are hingedly connected with each other and have corresponding mating surfaces surrounding a depression whereby when the mating surfaces are in contact the depressions form a cavity. The cavity completely encompasses the cap and a portion of the spout including a split ring that is firmly engaging the spout. The split ring being firmly attached to the spout and within the cavity prevents a thief from pulling up on the main body to frictionally engage and twist the cap off by engaging the inside edge of an inwardly extending flange of the main body while the main body is being rotated. The cavity is also tall enough to prevent a thief from pushing down to frictionally engage the cap with the main body. A lock securely holds the first and second hinging portions in a closed position.