A high-pressure tank liner manufacturing device includes: a heater to heat and melt end surfaces of liner halves; a heater transportation mechanism configured to slidably move the heater between a waiting position for the heater and a heating position for the heater; an elevation mechanism (drive mechanism) configured to drive a pair of the liner halves so as to cause the pair of liner halves to be relatively moved closer to each other or away from each other; and a parallelism adjustment mechanism configured to adjust parallelism of the end surfaces of the liner halves in accordance with the heater being slidably moved.