From ancient Greece to the present day, philosophers and scientists have endeavored to find the cause of gravitation (gravity) that causes objects to fall. Today, General Relativity reveals that gravitation is caused by curved space. However, its origin and generation mechanism of the force of gravitation is not well explained: the explanation of reason is insufficient why the force of gravitation is generated when space curves. Given a priori assumption that space as a vacuum has a physical fine structure like a continuum, it enables us to apply continuum mechanics to the so-called “vacuum” of space. The pressure field derived from the geometrical structure of space is newly obtained by applying both continuum mechanics and General Relativity to space. The apples on the Earth will not be pulled from the Earth and fall but will be pushed and fall in the direction of the Earth due to the pressure of the field in the curved space area around the Earth. This book describes the cause and mechanism of GRAVITATION.