To obtain 1,2,3,4-tetrahydro-3-isoquinolinecarboxylic acid of high optical purity by allowing phenylalanine or its derivative to react with a formaldehyde precursor in a solvent containing concentrated hydrochloric acid whose waste liquid treatment is easy and economic.

In a solvent of concentrated hydrochloric acid, (A) a phenylalanine derivative of high optical purity represented by formula I (R1 and R2 are each H, a 1-7C alkyl, a 1-7C alkoxy, OCH2O group, a halogen; R3 is H, a 1-6C alkoxy) is allowed to react with (B) a formaldehyde precursor, preferably formalin, trioxane, dialkoxymethane, or paraformaldehyde at 40-60°C for 10-60 hours to form (C) a compound of formula II. Then, the product is neutralized with a base such as ammonium hydroxide, potassium carbonate or sodium carbonate in hot water at 70-100°C to give (D) 1,2,3,4-tetrahydro-3- isoquinolinecarboxylic acid of formula III or its derivative. The concentrated hydrochloric acid used in this process is preferably recycled for reuse.