To improve heat stability, light fastness, tinting strength, and brightness in thermal transfer by using a specific methine dye.

A methine dye for use in thermal transfer, represented by the formula [wherein K represents a five- or six-membered carbocyclic or heterocyclic group; X represents N or C-Q1; Q1 to Q3 represent H, a 1-6C alkyl, benzyl, a 3-8C cycloalkyl, (substd.) phenyl, a 1-4C fluoroalkyl, a 1-6C alkoxy, benzyloxy, (substd.) phenoxy, a 1-6C alkylthio, a halogen, cyanom formylamino, R3, -CO-OR1, -CO-NHR1, -CP-NH-CO-R1, -CO-NH-CO-R4, -CO-NH-SO2R4, -NH-CO-R1, -NH-CO- OR1, -NH-CO-NR1R2, -NH-CS-OR1, -NH-CS-NR1R2, -NH-CO-R4, -NH-SO2-R1, -NH-SO2-R4, or -NH-SO2-NR1R2; R1 and R2 represents a 1-13C alkyl, a 3-8C cycloalkyl, or (substd.) phenyl; and R3 and R4 represent a five- or six-membered heterocyclic group].