PURPOSE: To control the operation of an oxygen manufacturing plant so as to reduce the manufacturing cost while satisfying the required amount of a customer by a method wherein the information of a contract with a power plant company and the manufacturing amount of respective time bands are prepared as a calendar information while the calendar information is compared with a present time to set the manufacturing amount automatically.

CONSTITUTION: The required amount of manufacturing in every time bands, considering charges of electricity in respective time bands and a divided time of respective time bands of week day, sunday and national holiday, nighttime and peak demand based on a power receiving contract with a power company, is stored into a calendar information memory 1. When a present time information B is taken into a comparing and judging means 3 from a clock device 2 under this condition, a comparing and judging means 3 compares the time information B with the calendar information A while the manufacturing amounts P, Q, R in respective time bands and kinds of products are extracted out of the calendar information A to store them into memories 4-6 corresponding to respective products. The values P-R are converted into control loop setting values S-U by set value converting means 7-9 while plant control means 10-12 control a control loop based on the set value of plant control means 10-12.