An infusion fluid warming apparatus (10; 40) for warming an infusion fluid to a predetermined temperature prior to supply of the infusion fluid into a living body through an infusion fluid tube, said apparatus comprising: a tube holding device (12, 14, 15, 20; 12, 14, 15, 42) detachably holding the infusion fluid tube; and a heat accumulator member generating a latent heat in phase change thereof from liquid phase to solid phase, said heat accumulator member being disposed in the tube holding device (12, 14, 15, 20; 12, 14, 15, 42) so as to warm the infusion fluid passing through the infusion fluid tube that is held by the tube holding device (12, 14, 15, 20; 12, 14, 15, 42) by the latent heat generated in the phase change from liquid phase to solid phase, characterized by that said tube holding device (12, 14, 15, 42) has an electric heater operable to heat the heat accumulator member so as to cause the phase change of the heat accumulator member from the solid phase to the liquid phase, said tube holding device (12, 14, 15, 20; 12, 14, 15, 42) has a tube holding member (20; 42) holding the infusion fluid tube in a predetermined curved shape, and wherein the accommodating bag is detachably held in close contact directly or indirectly with the tube holding member, said heat accumulator member includes, in addition to a heat accumulating material, a trigger member, operable to initiate the phase change of the heat accumulating material from the liquid phase to the solid phase, and a flexible accommodating bag fluid-tightly accommodating the heat accumulating material and the trigger member, and said accommodating bag is provided by a flexible sheet, and wherein said accommodating bag has a heat conducting fin including an end portion connected to a portion of the flexible sheet that is in close contact with the tube holding member (20; 42), and another end portion separated from the portion of the flexible sheet. a tube holding device (12, 14, 15, 20; 12, 14, 15, 42) detachably holding the infusion fluid tube; and a heat accumulator member generating a latent heat in phase change thereof from liquid phase to solid phase, said heat accumulator member being disposed in the tube holding device (12, 14, 15, 20; 12, 14, 15, 42) so as to warm the infusion fluid passing through the infusion fluid tube that is held by the tube holding device (12, 14, 15, 20; 12, 14, 15, 42) by the latent heat generated in the phase change from liquid phase to solid phase, characterized by that said tube holding device (12, 14, 15, 42) has an electric heater operable to heat the heat accumulator member so as to cause the phase change of the heat accumulator member from the solid phase to the liquid phase, said tube holding device (12, 14, 15, 20; 12, 14, 15, 42) has a tube holding member (20; 42) holding the infusion fluid tube in a predetermined curved shape, and wherein the accommodating bag is detachably held in close contact directly or indirectly with the tube holding member, said heat accumulator member includes, in addition to a heat accumulating material, a trigger member, operable to initiate the phase change of the heat accumulating material from the liquid phase to the solid phase, and a flexible accommodating bag fluid-tightly accommodating the heat accumulating material and the trigger member, and said accommodating bag is provided by a flexible sheet, and wherein said accommodating bag has a heat conducting fin including an end portion connected to a portion of the flexible sheet that is in close contact with the tube holding member (20; 42), and another end portion separated from the portion of the flexible sheet.