Norvaline derivative of the formula [I] or pharmaceutically acceptable salt thereof, method for preparing the same, pharmaceutical composition containing the same, and use of the compound for inhibiting transporting activity of glycine transporter type 2 (GlyT2) wherein X is —CH2—, —O—, —S— or single bond; Ar is optionally substituted aryl or lower cycloalkyl; n is 0 to 2; R1 and R2 are (i) each is hydrogen or lower alkyl; (ii) R1 and R2 are combined to form lower alkylene; or (iii) R1 is hydrogen or lower alkyl and R2 is combined with R4 or R6 to form lower alkylene; R3 and R4 are (i) each is hydrogen or lower alkyl; (ii) R3 and R4 are combined to form lower alkylene; or (iii) R3 is hydrogen or lower alkyl and R4 is combined with R2 or R6 to form lower alkylene; R is  or —OR7; R5 and R6 are (i) each is optionally substituted lower alkyl, or hydrogen; (ii) R5 and R6 are combined to form aliphatic 5- to 6-membered heterocyclic group; or (iii) R5 is optionally substituted lower alkyl or hydrogen and R6 is combined with R2 or R4 to form lower alkylene; R7 is lower alkyl.