[Problem] To provide nanoparticles for photo dynamic therapy, the nanoparticles being capable of building a system which can accurately determine sites of malignant tumors and the like, especially, metastatic spinal tumors that should be surgically removed by visualizing the sites specifically and with sufficient emission intensity, and which can effectively process the determined sites. [Solution] Nanoparticles for photo dynamic therapy comprising an amphiphilic block polymer (A1) containing a hydrophilic block chain having not less than 20 sarcosine units and a hydrophobic block chain having not less than 10 lactic acid units, and a near infrared fluorescent dye (B) which causes cytotoxicity by laser irradiation.
