The organic crystal consisted of a methylene-bridged azobenzene dimer possessing long-chain alkoxy groups melts by UV light irradiation. This characteristic phase transition is named as the photoinduced crystal-melt transition and starts from the trans-cis photoisomerization of azobenzene moieties at a solid state. Herein, elucidation of detailed mechanism of the photoinduced crystal-melt transition in the crystal of the above target compound by single crystal X-ray structure analysis is described. In this crystal structure, azobenzene moieties are aligned via intermolecular π…π interactions for constructing three-dimensional periodic structure as a crystal. On the other hand, long-chain alkoxy groups are disturbed due to their thermal motion. This thermal motion of them mitigates the mechanical stress from geometrical change accompanying with trans-cis photoisomerization of azobenzene moieties and leads formation of an overall isotropic melt phase with sacrificing π…π interactions in the crystalline phase. Visualizing these structural features playing a role for photoinduced crystal-melt transition rationalizes phase transition induced by solid-state photoreaction with proposing a phase diagram.