Provided is a prophylactic/therapeutic agent for influenza viral infection that is effective not only before and at an early stage of infection with influenza virus but also at an intermediate or late stage of the infection and is highly safe for human bodies. A prophylactic/therapeutic drug for influenza viral infection comprising, as active ingredients, 5-aminolevulinic acid (5-ALA), a derivative thereof or a salt of the 5-ALA or the derivative, and an iron compound is prepared. This prophylactic/therapeutic agent can be used for ameliorating (preventing) depression in food consumption, water consumption and body weight, for ameliorating (decreasing) increase in ketone body levels in blood that may otherwise cause ketosis, for ameliorating (preventing) depression in ATP levels in blood, or for ameliorating (increasing) a survival rate and depression in a body surface temperature.