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 ´-aminolevulinic acid represented by formula (I), a derivative thereof and a salt thereof: €ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒR 2 R 1 NCH 2 COCH 2 CH 2 COR 3 €ƒ€ƒ€ƒ€ƒ€ƒ(I) wherein R 1 and R 2 each independently represents hydrogen, alkyl, acyl, alkoxycarbonyl, aryl or aralkyl and R 3 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.
