The present invention provides a compound having an mPGES-1 inhibitory activity and useful for the prophylaxis or treatment of pain, rheumatism, osteoarthritis, fever, Alzheimer' s disease, multiple sclerosis, arteriosclerosis, glaucoma, ocular hypertension, ischemic retinal disease, systemic scleroderma and/or cancer including colorectal cancer. The present invention relates to a compound of formula [I-a], [I-b] or [I-c] or a pharmaceutically acceptable salt thereof : wherein each symbol is as defined in the specification.