The present invention provides a compound represented by thefollowing formula (1), a pharmaceutically acceptable salt thereof, or asolvatethereof.wherein:R1 and R2 are each separately selected from the group consisting ofhydrogen, 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(iii), and (iv):[Formula 2]in the formula (ii), R a is alkyl；R4 and R5 are each separately selected from the group consisting ofhydrogen, hydroxy, alkoxy, haloalkoxy, halohydroxyalkoxy, and aminoalkyl；andrepresents a double bond or a triple bond. The above compound can be usedas a molecular probe for imaging tau proteins that accumulate in the brain.
