A loosening prevention device (1) for a plug includes: a first sleeve (11) engaged with and surrounding a first plug (70) attached to a lid of a liquid tank (60); a second sleeve (21) engaged with and surrounding a second plug (80) attached to the lid; and a connection means (24) for connecting and fixing the first sleeve and the second sleeve with each other. At least one flat surface (75, 85) is formed on an outer circumferential surface (74, 84) of both or either one of the first plug and the second plug, and at least one flat surface (12, 22) in surface contact with the flat surface of both or either one of the first plug and the second plug is formed on an inner circumferential surface of both or either one of the first sleeve and the second sleeve. With this arrangement, a plug for supplying pressurized-gas and a plug for discharging liquid can be firmly held.