The present invention provides a compound represented by the following formula (I), a pharmaceutically acceptable salt thereof, or a solvate thereof:




wherein:



R1 and R2 are each separately selected from the group consisting of hydrogen, alkyl, alkenyl, acyl, and hydroxyalkyl;
R3 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), Ra is alkyl;
R4 and R5 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.