There is provided a pharmaceutical drug, particularly a novel compound useful for prophylaxis or a therapeutic treatment of various diseases involving infections with viruses of the herpesvirus family, specifically various herpesvirus infections such as varicella (chicken pox) via varicella zoster virus, varicella zoster via recurrent infection with latent varicella zoster virus, herpes labialis and herpes encephalitis via HSV-1 and genital herpes via HSV-2 infection. An N-{2-[(4-substituted phenyl)amino]-2-oxoethyl}tetrahydro-2H-thiopyran-4-carboxamide derivative, of which phenyl group is substituted at position 4 with a specific 5- or 6-membered heteroaryl group, and a salt thereof have an effective anti-virus activity, and the oral administration thereof at a low dose enabled the therapeutic treatment of the above diseases.
