A bolt fixing device of a jet pump beam for fixing a head bolt of a jet pump beam pressing an inlet mixer from vertically above, the inlet mixer being arranged on an upper portion of a riser of the jet pump of a boiling water reactor and forming a reversing channel of cooling water, the bolt fixing device comprising: a lock cap arranged to fit to a polygonal head portion of the head bolt, arranged to be movable in a vertical axis direction, and arranged to be integrally rotatable with the head bolt; that is a body housing that accommodates the lock cap and that is suitable for being arranged on an upper surface of the jet pump beam; and an elastic member that is arranged in the body housing below the lock cap for applying vertically upward elastic force to the lock cap, the lock cap being able to be pushed up by the elastic force of the elastic member so that a plurality of upper surface teeth circumferentially formed on an upper surface of a flange of the lock cap mesh with a plurality of lower surface teeth circumferentially formed on a lower surface of a locking flange of the body housing, whereby the head bolt of the jet pump beam is fixed wherein the upper surface teeth of the lock cap and the lower surface teeth of the body housing are formed in serrated shapes and include vertical surfaces extending vertically that face each other in contact with each other when the head bolt is rotated to be loosened, and include inclined surfaces that face each other in contact with each other when the head bolt is rotated to be tightened, and a pitch of the upper surface teeth of the lock cap and a pitch of the lower surface teeth of the body housing determine the maximum amount of rotation of the head bolt in the loosening direction whilst being in a tolerance range of the tightening force of the head bolt to ensure the upper surface teeth and lower surface teeth mesh. a lock cap arranged to fit to a polygonal head portion of the head bolt, arranged to be movable in a vertical axis direction, and arranged to be integrally rotatable with the head bolt; that is a body housing that accommodates the lock cap and that is suitable for being arranged on an upper surface of the jet pump beam; and an elastic member that is arranged in the body housing below the lock cap for applying vertically upward elastic force to the lock cap, the lock cap being able to be pushed up by the elastic force of the elastic member so that a plurality of upper surface teeth circumferentially formed on an upper surface of a flange of the lock cap mesh with a plurality of lower surface teeth circumferentially formed on a lower surface of a locking flange of the body housing, whereby the head bolt of the jet pump beam is fixed wherein the upper surface teeth of the lock cap and the lower surface teeth of the body housing are formed in serrated shapes and include vertical surfaces extending vertically that face each other in contact with each other when the head bolt is rotated to be loosened, and include inclined surfaces that face each other in contact with each other when the head bolt is rotated to be tightened, and a pitch of the upper surface teeth of the lock cap and a pitch of the lower surface teeth of the body housing determine the maximum amount of rotation of the head bolt in the loosening direction whilst being in a tolerance range of the tightening force of the head bolt to ensure the upper surface teeth and lower surface teeth mesh.