The acid-doping to the tropone-containing conjugated polymer (TCP) was demonstrated. The color of the chloroform solution of TCP changed from orange to dark red by the addition of trifluoroacetic acid, while no color change was observed in the experiment using the low molecular weight model compound. The fluorescence from TCP was completely quenched with the excess amount of a protic acid. The quasi-reversible protonation-deprotonation cycle was confirmed when triethylamine was dropped into the proton-doped TCP.