A hazardous substance treatment agent has a retainer and a transition metal oxide. The retainer has the specificity of retaining only a specific hazardous substance, such as a virus, a bacterium or a toxin, that has become mixed in or has the possibility of becoming mixed in a treatment subject, such as blood, that is in at least either a liquid or gaseous phase. By a photocatalytic action, the transition metal oxide inactivates said hazardous substance retained by the retainer.