Electrolyte and lithium ion secondary battery

An electrolyte, which comprises (a) a compound having at least one methylene group adjacent to an oxygen atom in the molecule, (b) a compound represented by the following formula (1), wherein R is and (c) an electrolytic salt, compounds (a) and (b) having been polymerized with each other, exhibits a high ionic conductivity, and a lithium ion secondary battery incorporating said electrolyte exhibits superior charge and discharge characteristics.