Provided is a molten salt composition, which is appropriately usable as an electrolyte for secondary batteries and has no definite melting point, obtained by mixing two or more kinds of molten salts that are usable as an electrolyte for secondary batteries, in particular, the aforesaid molten salt composition characterized in that two types of molten salts comprise cations that have different ionic diameters and the composition ratio thereof falls within such a composition ratio range as allowing the molten salt composition to show no melting point. Also provided is a secondary battery using the molten salt composition as an electrolyte, when temperature drops, said secondary battery not suddenly becoming unusable but being kept in a usable state.