Provided is an active energy ray curable undercoat agent for forming an undercoat layer in a metal film-attached base material comprising a base material, an undercoat layer, and a metal film in this order. The active energy ray curable undercoat agent contains a first compound having three or more (meth)acryloyl groups, a second compound, and a photopolymerization initiator, wherein: the second compound contains at least one type selected from the group consisting of a compound having a silsesquioxane skeleton, a metal compound having an organic group, a triazinethiol compound, and a silane coupling agent having two or more alkoxysilyl groups and two or more reactive functional groups; and the content of the first compound is 70 mass percent or more in 100 mass percent of a nonvolatile component of the active energy ray curable undercoat agent.
