The invention provides an ink that inhibits lowering of ejection stability caused by a precipitate derived from a self-dispersible type monoazo pigment and also inhibits deterioration of a liquid contact surface of a heater portion of a recording head. The ink is an ink for ink jet recording system employing an thermal energy, containing a monoazo pigment, a resin and a surfactant represented by the following formula (1). A proportion of a molecular weight of an ethylene oxide group moiety occupying the surfactant is 10% or more and 85% or less, the monoazo pigment is a self-dispersible pigment to the particle surface of which at least one hydrophilic group selected from the group consisting of —COOM, —SO3M, —PO3HM and —PO3M2 is bonded directly or through another atomic group, and the acid value of the resin is 240 mg KOH/g or less.