The present invention provides a compound represented by the following formula (I), a pharmaceutically acceptable salt thereof, or a solvate thereof. wherein: R 1 and R 2 are each separately selected from the group consisting of hydrogen, alkyl, alkenyl, acyl, and hydroxyalkyl； R 3 is hydrogen or halogen； ring A is a benzene ring or a pyridine ring； ring B is selected from the group consisting of the following formulas (i), (ii), (iii), and (iv): in the formula (ii), R a is alkyl； R 4 and R 5 are each separately selected from the group consisting of hydrogen, hydroxy, alkoxy, haloalkoxy, halohydroxyalkoxy, and aminoalkyl； and represents a double bond or a triple bond. The above compound can be used as a molecular probe for imaging tau proteins that accumulate in the brain.
