Abstract: [Problem] To provide a nuclear magnetic resonance diagnostic agent that has a lower toxicity to organisms and reduced side effects and yet has a site specificity toward a specific cell, tissue, organ, etc. [Solution] When ALA or an ALA derivative is administered in vivo, a metabolite thereof is accumulated in a specific cell, tissue, organ, etc. Focusing on this phenomenon, a nuclear magnetic resonance analysis was performed on a site wherein the metabolite of ALA that had been administered in vivo would be possibly accumulated. As a result, it was surprisingly found that ALA and an ALA derivative are useful as a diagnostic agent whereby the aforesaid problem can be solved.