NOVELTY - An ionic liquid contains a phosphonium cation component (1) and an anion component.    USE - As electrolyte liquid of electrical-storage device and lithium secondary battery, electrical-double-layer capacitor, dye-sensitized solar cell, and fuel cell, and for reaction solvent (all claimed).    ADVANTAGE - The ionic liquid has low viscosity, favorable electric conductivity, and excellent electrochemical stability. The ionic liquid exhibits stable liquid state at room temperature vicinity.    DETAILED DESCRIPTION - An ionic liquid contains a phosphonium cation component of formula (1), and an anion component.    R1-R9=H, 1-30C linear or branched alkyl, 2-30C linear or branched alkenyl having double bond(s), 2-30C linear or branched alkynyl having triple bond(s), (un)saturated cycloalkyl, aryl, or heterocyclic group;    X=sulfur, oxygen or carbon;and    dotted line=conjugated structure.    A halogen, CN group, or NO groups can substitute H of R1-R9. Arbitrary substituents of R1-R9 may form a cyclic structure. -O-, -C(O)-, -C(O)O-, -S-, -S(O)-, -SO2-, -SO3-, -N=, -N=N-, -NH-, -NR'-, -N(R')2, -PR', -P(O)R'-, -P(O)R'-O-, -O-P(O)R'-O-, and/or -P(R')2=N- (R' is 1-10C linear or branched alkyl, alkyl partially or completely substituted with fluorine atom, (un)saturated cycloalkyl, optionally substituted phenyl, or optionally substituted heterocyclic group) can substitute carbon atom in R1-R9. R8 and R9 exist only when X is carbon. When X is carbon, X, R1, R8 and R9 may form a cyclic structure of (un)saturated state. INDEPENDENT CLAIMS are included for the following:    (1) electrical-storage device containing the ionic liquid as an electrolyte liquid;    (2) lithium secondary battery containing the ionic liquid;    (3) electrical-double-layer capacitor containing the ionic liquid;    (4) dye-sensitized solar cell containing the ionic liquid;    (5) fuel cell containing the ionic liquid;    (6) reaction solvent containing the ionic liquid; and    (7) manufacture of ionic liquid, which involves alkylating an organophosphorus compound of formula (2).    R2-R9,X=as mentioned above.