It is intended to provide highly safe antitumor agents which exhibit an antitumor effect on human remote tumors such as metastatic tumors too and by which an antitumor immune reaction enabling an immune therapy for cancer can be induced, tumor immunity inducers, T cell activators, dendritic cell activators, a method of treating cancer using the same, etc. Inactivated herpes simplex virus (inactivated HSV), herpes simplex virus glycoprotein D (HSVgD), etc. are employed as the active ingredients of antitumor agents, tumor immunity inducers, T cell activators or dendritic cell activators. As a specific example of the treatment for the above-described inactivation, citation may be made of a combination of UV-irradiation using ultraviolet light at 254 nm at 4 J/m.sup.2 for 30 minutes with heating at 56.degree. C. for 30 minutes.