A mold unit comprising: a female mold and a male mold, which are closely engaged with each other in a selective manner to define a cavity having a desired shape, and are disengaged from each other, wherein the female and male molds are supported so as to be movable relative to each other in the direction of a common axis, and so as to be rotatable relative to each other about the axis; a first member that moves integrally with the male mold; and a second member that moves integrally with the female mold, wherein the first and second members are rotatably and axially movable relative to each other, and have a closest relative axial-position to each other that varies between a prevention position, in which the male mold is prevented from being brought into close engagement with the female mold, and a permission position, in which the male mold is permitted to be brought into close engagement with the female mold, depending on a relative rotational-position of the first and second members, the relative rotational-position of the first and second members varies depending on an the inertial force acting on the male mold in the rotational direction of the male mold according to acceleration or deceleration of the rotation, and the relative axial-position of the first and second members varies depending on an the axial centrifugal-force acting on the male mold according to an orbiting speed of the mold unit, to thereby allow the male mold to selectively take positions, in which the male mold is selectively prevented from being brought into close engagement with the female mold and is permitted to be brought into close engagement with the female mold. a female mold and a male mold, which are closely engaged with each other in a selective manner to define a cavity having a desired shape, and are disengaged from each other, wherein the female and male molds are supported so as to be movable relative to each other in the direction of a common axis, and so as to be rotatable relative to each other about the axis; a first member that moves integrally with the male mold; and a second member that moves integrally with the female mold, wherein the first and second members are rotatably and axially movable relative to each other, and have a closest relative axial-position to each other that varies between a prevention position, in which the male mold is prevented from being brought into close engagement with the female mold, and a permission position, in which the male mold is permitted to be brought into close engagement with the female mold, depending on a relative rotational-position of the first and second members, the relative rotational-position of the first and second members varies depending on an the inertial force acting on the male mold in the rotational direction of the male mold according to acceleration or deceleration of the rotation, and the relative axial-position of the first and second members varies depending on an the axial centrifugal-force acting on the male mold according to an orbiting speed of the mold unit, to thereby allow the male mold to selectively take positions, in which the male mold is selectively prevented from being brought into close engagement with the female mold and is permitted to be brought into close engagement with the female mold.