A contrast agent for photoacoustic imaging, comprising: a complex including albumin covalently bound to at least one organic dye that absorbs light in the near-infrared wavelength region, wherein the organic dye that absorbs light in the near-infrared wavelength region is represented by formula: B-B' wherein in formula, B represents formula (i) or (ii); B' represents any one of formulae (iii) to (vi), wherein *'s in formulae (i) and (ii) each bind to B' of formula; and *'s in formulae (iii) to (vi) each bind to B of formula: wherein in formulae (i) and (ii), Z's each represent a hydrogen atom, a sulfonic group, or a cyclic aromatic ring selected from the group consisting of a benz[e]indole ring, a benz[f]indole ring, and a benz[g]indole ring formed together with an indole ring bound to a corresponding one of Z's; and the hydrogen atoms of the cyclic aromatic ring each may be replaced with an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, or a sulfonic group, wherein in formulae (i) and (ii), R 1 's each represent an alkyl group having 1 to 10 carbon atoms or -(CH 2) b -SO 3 - wherein b represents an integer of 1 to 10; when R 1 's each represent an alkyl group, a halide ion or an organic acid ion may be contained as a counter ion; and R 2 's and R 3 's each independently represent a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, -(CH 2) b -SO 3 - wherein b represents an integer of 1 to 10, or -(CH 2) b -SO 3 X wherein b represents an integer of 1 to 10, and X represents sodium, potassium, ammonium, triethylammonium, lysine, or arginine, wherein in formulae (i) and (ii), a's each represent an integer of 1 to 10; and n's each represent 2 or 3, and wherein in formula (ii), R 4 represents an alkyl group having 1 to 10 carbon atoms or -(CH 2) b -SO 3 X wherein b represents an integer of 1 to 10, and X represents sodium, potassium, ammonium, triethylammonium, lysine, or arginine: *-OH (vi) characterized in that: on average in the contrast agent, a number of molecules of the organic dye covalently bound to the albumin of the complex is 1.6 or more. a complex including albumin covalently bound to at least one organic dye that absorbs light in the near-infrared wavelength region, wherein the organic dye that absorbs light in the near-infrared wavelength region is represented by formula: B-B' wherein in formula, B represents formula (i) or (ii); B' represents any one of formulae (iii) to (vi), wherein *'s in formulae (i) and (ii) each bind to B' of formula; and *'s in formulae (iii) to (vi) each bind to B of formula: wherein in formulae (i) and (ii), Z's each represent a hydrogen atom, a sulfonic group, or a cyclic aromatic ring selected from the group consisting of a benz[e]indole ring, a benz[f]indole ring, and a benz[g]indole ring formed together with an indole ring bound to a corresponding one of Z's; and the hydrogen atoms of the cyclic aromatic ring each may be replaced with an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, or a sulfonic group, wherein in formulae (i) and (ii), R 1 's each represent an alkyl group having 1 to 10 carbon atoms or -(CH 2) b -SO 3 - wherein b represents an integer of 1 to 10; when R 1 's each represent an alkyl group, a halide ion or an organic acid ion may be contained as a counter ion; and R 2 's and R 3 's each independently represent a hydrogen atom, an alkyl group having 1 to 10 carbon atoms, an alkoxy group having 1 to 10 carbon atoms, -(CH 2) b -SO 3 - wherein b represents an integer of 1 to 10, or -(CH 2) b -SO 3 X wherein b represents an integer of 1 to 10, and X represents sodium, potassium, ammonium, triethylammonium, lysine, or arginine, wherein in formulae (i) and (ii), a's each represent an integer of 1 to 10; and n's each represent 2 or 3, and wherein in formula (ii), R 4 represents an alkyl group having 1 to 10 carbon atoms or -(CH 2) b -SO 3 X wherein b represents an integer of 1 to 10, and X represents sodium, potassium, ammonium, triethylammonium, lysine, or arginine: *-OH (vi) characterized in that: on average in the contrast agent, a number of molecules of the organic dye covalently bound to the albumin of the complex is 1.6 or more.