PURPOSE: To obtain oxyguanidine useful as an intermediate for herbicides industrially advantageously by reacting isothiourea or its hydrogen halide which is novel in some case with hydroxylamine.

CONSTITUTION: An isothiourea or a hydrogenhalide thereof of formula I [wherein R2 is selected from a set of H, a halogen, a hydroxyalkyl, amino, an alkyl, an alkoxyl, etc.; R5 is an alkyl or an aralkyl; X is N or CH; and Z is N or CR4 (wherein R4 is R2)] is reacted with a hydroxylamine of formula II (wherein R1 is an alkyl, an alkenyl, an aryl or the like) at 0-150°C to obtain a compound of formula III. Among compounds of formula I, a compound of formula IV (wherein A1 is R5; A2 is a halogen or the like; A3 is H or A2; and Q is X) (except N-(4,5-dimethoxy-S-triazin-2-yl)-S-methylisourea) is novel. A compound of formula V is reacted with an alkylating agent of the formula: A1-X1 to obtain a compound of formula IV.