Provided is a novel process for selectively producing the E-geometrical isomer or Z-geometrical isomer of an oximino compound useful as a medicine or agrochemical. In the process, a mixture of geometrical isomers of an oximino compound represented by formula (EZ)-1 is mixed with an acidic compound to produce the E-isomer of the oximo compound represented by the formula (E)-1 or the Z-isomer of the oximino compound represented by the formula (Z)-1 in high yields with high stereoselectivity.