NEW MATERIAL:The titled derivative expressed by formula I (R1 is cyano, lower alkoxycarbonyl, carboxyl or carbamoyl; R2 and R3 are H or protecting group; R4 is H, protecting group or phosphoric acid group).

EXAMPLE: 5-Cyanomethyl-1-β-D-arabinofuranosyluracil expressed by formula II.

USE: An antiviral agent.

PROCESS: A 5-halogenomethyl-1-β-D-arabinofuranosyluracil (derivative) expressed by formula III is reacted with a cyanogenating agent, e.g. expressed by the formula M1CN [M1 is alkali (earth) metal], preferably sodium cyanide or potassium cyanide, preferably in a polar solvent, e.g. dimethylformamide, at 0°C to room temperature for 30minW5hr to afford the exemplified compound expressed by formula II.