A very-high pressure generator of construction such that the lower and upper guide blocks of the generator are each configured so as to form a pyramidal recess on the bottom surface and an upside-down pyramidal recess on the top surface accurately symmetrically, their pyramidal slopes given one and the same angle of inclination, and are prevented from being deformed under high pressure not by enlarging the guide blocks and the press, but by making the support conditions of all the anvils of the generator uniform, the positions of the anvils can easily be adjusted, and therefore the generator is capable of pressurizing a pressure-transmitting medium into the shape of a desired cube accurately. Each of the lower and upper guide blocks has a pyramidal recess in its bottom surface and an upside-down pyramidal recess in its top surface and is symmetric with respect to its horizontal center plane. Each of lower and upper base blocks has a lower upside-down pyramidal portion and an upper pyramidal portion. The upper base block is disposed at the center of the lower pyramidal recess of the upper guide block. An upper downward pyramidal block is disposed between the press frame and the upper upside-down pyramidal recess of the upper guide block.