According to the present invention, a tumor diagnostic agent which carries out early stage detection, monitoring of therapeutic effect and prognostic diagnosis of all tumors The tumor diagnostic agent of the present invention is a tumor diagnostic agent comprising δ-aminolevulinic acid represented by formula (I), a derivative thereof and a salt thereof:R2R1NCH2COCH2CH2COR3 (I)wherein R1 and R2 each independently represents hydrogen, alkyl, acyl, alkoxycarbonyl, aryl or aralkyl； and R3 represents hydroxy, alkoxy, acyloxy, alkoxycarbonyloxy, aryloxy, aralkyloxy or amino, which is used for diagnoses by measuring porphyrins in a sample collected from the inside or outside of the body after its administration.
