Phospholipid derivative

123 13132 A phospholipid derivative represented by the following formula (I) wherein RCO and RCO independently represent an acyl group; Rrepresents hydrogen atom, or a hydrocarbon group; symbol "a" represents an integer of 0 to 4; symbol "b" represents 0 or 1, provided that when a is 0, b is 0; X represents hydrogen atom, an alkali metal atom, an ammonium, or an organic ammonium; AO and AO represent an oxyalkylene group containing oxyethylene group, wherein the ratio of the oxyethylene group to the oxyalkylene group in AO and AO is 0.5 or larger in terms of a weight ratio; AO represents an oxyalkylene group; symbols "m" and "q" represent an average molar number of added oxyalkylene groups; and symbol "n" represent an average molar number of added oxyalkylene groups; provided that m, n and q satisfy the following conditions: 5≦m≦600, 1≦n≦45, 0≦q≦200, 10≦m+n+q≦600, 0.04≦n/(m+n+q), and q/(m+n+q)≦0.8, which can thicken the water shell of liposome surface by suppressing the spreading of the polyalkylene oxide structure on the surface and thus increase stability of the liposome