Stabilized benzimidazole derivative and composition

A stabilized physiologically active benzimidazole derivative having the formula (I): wherein R¹ is hydrogen atom, an alkyl group having 1 to 8 carbon atoms, a fluoroalkyl group having 1 to 6 carbon atoms, a cycloalkyl group, phenyl group or an aralkyl group, R² is hydrogen atom or a lower alkyl group, or R¹ and R² together with the adjacent nitrogen atom form a ring, and each of R3a, R3b, R4a, R4b and R4c independ­ently is hydrogen atom, a halogen atom, a flouroalkyl group having 1 to 6 carbon atoms, a lower alkyl group, a lower alkoxy group, a lower alkoxycarbonyl group or an amino group. The stabilized benzimdazole derivative is in amorphous form or present in contact with a basic material.