An object of the present invention is to provide a probe for imaging a β-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, R1, R2, R3, R4, R5, R6, Ra and Rb are as defined in the present description) or a pharmaceutically acceptable salt or solvate thereof.