Abstract: Provided are: a method for controlling the decoloration reaction rate and color density of a fast light-modulating material to levels suitable for practical use; and a fast light-modulating material having a decoloration reaction rate and a color density which are suitable for practical use. A polymer obtained by polymerizing a paracyclophane-bridged hexaarylbisimidazole compound having a radical-polymerizable group, and a copolymer obtained by copolymerizing the compound with a (meth)acrylic acid compound or a sensitizer having a radical-polymerizable group can achieve controlled decoloration reaction rates, photosensitivity and color tones.