PROBLEM TO BE SOLVED: To provide composite particles capable of being used not only as an MRI imaging agent by shortening the relaxation time of water, but also as a photoacoustic mammography by generating a photoacoustic signal, a method for producing the same, and the MRI imaging agent and a multi-functional molecular imaging probe, containing the composite particles.SOLUTION: There are provided the composite particles obtained by forming the film consisting of an amphipathic polymer compound on the surface of gadolinium oxide-containing particles, the MRI-imaging agent containing the composite particles, the imaging probe for the photoacoustic mammography containing the composite particles, and the method for producing the composite particles by covering the amphipathic polymer compound on the surface of the gadolinium oxide-containing particles. In addition, the method for controlling the particle diameter of the composite particles is provided by dispersing the gadolinium oxide-containing particles in water, adjusting the particle diameter of the gadolinium oxide-containing particles and then covering the amphipathic polymer compound on the surface of the gadolinium oxide-containing particles.
