PROBLEM TO BE SOLVED: To provide a means which can liquefy boil-off gas generating from a liquid hydrogen storage tank while utilizing cold energy effectively, and recycle it as liquid hydrogen.SOLUTION: A liquid hydrogen production device HS is provided with a refrigeration cycle unit R in which circulating hydrogen functions as a refrigerant, and a liquid hydrogen generation unit P for generating liquid hydrogen by cooling high-pressure raw material hydrogen by means of the refrigeration cycle unit R and by adiabatically expanding the raw material hydrogen by means of a Joule-Thomson valve 12. A first and second heat exchanger E1, E2 are disposed along the refrigeration cycle unit R and the liquid hydrogen generation unit P. The liquid hydrogen production device HS is provided with a boil-off gas processing mechanism for generating liquid hydrogen by re-liquefying the boil-off gas generated in a liquid hydrogen storage tank on a liquid hydrogen transport vessel 16. The boil-off gas is introduced into a hydrogen circulation path 1 at a section at which circulating hydrogen having an extremely low temperature flows, and the excessive circulating hydrogen generated therefrom is discharged to a raw material hydrogen path 11 from a section at which the circulating hydrogen is at room temperature.