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.