A remarkable deuterium isotope effect on the photochromism of 3,3'-diaryl-3H,3'H-[2,2']biindenylidene-1,1'-dione (2) in the solid state has been found. The yellow crystals of 2 and 2-d(2) were immediately changed to reddish purple crystals upon photoirradiation in the solid state. The reddish purple crystals of 2-d(2) returned back to the original yellow crystals under dark for 72 h, whereas that of 2 faded within 2 h.