A sealant for an electrochemical device contains (A) a polyisobutylene polymer and (B) a conjugated diene polymer. The sealant contains the (A) polyisobutylene polymer in a proportion of more than 60 mass % and not more than 98 mass % and the (B) conjugated diene polymer in a proportion of not less than 2 mass % and less than 40 mass % relative to the total mass of the (A) polyisobutylene polymer and the (B) conjugated diene polymer.