One of the purposes of the present invention is to provide a probe for imaging a protein having a β-sheet structure for the purpose of diagnosing conformation diseases, particularly diseases in which the accumulation of tau protein in the brain is observed as the main characteristic symptom (tauopathy) such a Alzheimers disease.Another one of the purposes of the present invention is to provide a compound which has high specificity to tau, enables the imaging of tau with good sensitivity, has high brain migration properties, is accumulated in bones in a minute amount or is not accumulated in bones, and has low toxicity or does not have toxicity.The present invention achieves the above-mentioned purposes by providing a compound represented by formula (I) (wherein A, R
