A boil-off gas processing apparatus which reliquefies a boil-off gas generated within a liquefied gas tank storing a liquefied gas and returns the reliquefied gas to an interior of the liquefied gas tank , the boil-off gas processing apparatus comprising: a boil-off gas discharge line , which includes a boil-off gas discharge pipe insertable into an upper region of the interior of the liquefied gas tank , and the boil-off gas discharge line being configured to discharge the boil-off gas collected in the upper region of the interior of the liquefied gas tank , when mounted, from the liquefied gas tank to outside via the boil-off gas discharge pipe ; and a boil-off gas reliquefaction line , which includes a boil-off gas return pipe , which branches off from the boil-off gas discharge line and is insertable into the interior of the liquefied gas tank and immersable in the liquefied gas when the liquefied gas tank stores the liquefied gas , characterized in that the boil-off gas discharge line includes a compressor configured to discharge the boil-off gas or to increase pressure of the boil-off gas , the boil-off gas reliquefaction line further includes a float type vapor trap configured to condense and trap the boil-off gas transferred from the boil-off gas return pipe , temporarily store the boil-off gas as a liquid, and when the liquid stored reaches a certain amount, open to release the liquid into the liquefied gas , the float type vapor trap including a body , a float type on-off valve capable of vertically sinking and floating within the body , a liquid discharging orifice through which the liquefied gas is discharged, and a discharge port from which the discharged liquid is released to outside, the boil-off gas reliquefaction line maintains a pressure necessary for reliquefaction of the boil-off gas by the float type vapor trap , and a portion of the boil-off gas return pipe which is immersed in the liquefied gas when the liquefied gas tank stores the liquefied gas has a length sufficient to be able to release an amount of heat necessary for reliquefaction of the boil-off gas . a boil-off gas discharge line , which includes a boil-off gas discharge pipe insertable into an upper region of the interior of the liquefied gas tank , and the boil-off gas discharge line being configured to discharge the boil-off gas collected in the upper region of the interior of the liquefied gas tank , when mounted, from the liquefied gas tank to outside via the boil-off gas discharge pipe ; and a boil-off gas reliquefaction line , which includes a boil-off gas return pipe , which branches off from the boil-off gas discharge line and is insertable into the interior of the liquefied gas tank and immersable in the liquefied gas when the liquefied gas tank stores the liquefied gas , characterized in that the boil-off gas discharge line includes a compressor configured to discharge the boil-off gas or to increase pressure of the boil-off gas , the boil-off gas reliquefaction line further includes a float type vapor trap configured to condense and trap the boil-off gas transferred from the boil-off gas return pipe , temporarily store the boil-off gas as a liquid, and when the liquid stored reaches a certain amount, open to release the liquid into the liquefied gas , the float type vapor trap including a body , a float type on-off valve capable of vertically sinking and floating within the body , a liquid discharging orifice through which the liquefied gas is discharged, and a discharge port from which the discharged liquid is released to outside, the boil-off gas reliquefaction line maintains a pressure necessary for reliquefaction of the boil-off gas by the float type vapor trap , and a portion of the boil-off gas return pipe which is immersed in the liquefied gas when the liquefied gas tank stores the liquefied gas has a length sufficient to be able to release an amount of heat necessary for reliquefaction of the boil-off gas . <EOS>