An object of the present invention is to provide a probe for imaging a ²-sheet structure protein which can be used for the diagnosis of conformational diseases, particularly disease (tauopathy) having a cardinal symptom such as intracerebral accumulation of tau protein, for example, Alzheimer's disease. Another object of the present invention is to provide a compound which is highly specific to tau and can image tau with satisfactory sensitivity, and also has high brain transition, low or non-recognized bone-seeking properties and low or non-recognized toxicity. According to the present invention, the above problems are solved by providing a compound of a formula I (wherein A, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R a and R b are as defined in the present description) or a pharmaceutically acceptable salt or solvate thereof.
