A method for separating air by cryogenic distillation in an apparatus comprising: i) a first set of distillation columns comprising at least a first column operating at a first pressure, called high pressure, and a second column operating at a second pressure lower than the first pressure, the head of the first column being thermally connected to the bottom of the second column by means of a reboiler-condenser, a first purification unit and a first heat exchange line, and ii) a second set of distillation columns comprising at least a third column operating at a third pressure lower than the first pressure and a fourth column operating at a fourth pressure lower than the first, second and third pressures, the third column head being thermally connected to the bottom of the fourth column by a reboiler-condenser, a second purification unit (E') and a second exchange line heat, in which compressed air is sent to the first purification unit, the cleaned air is sent from the first purification unit to the first exchange line, cooled air is sent from the first heat exchange line to the first column, compressed air is sent to the second purification unit, cleaned air is sent from the second purification unit to the second line exchange, and cooled air is sent from the second exchange line to the third column operating at the third pressure characterized in that: - a head fluid is sent from the third column to the second column or the head gas from the third column is mixed with a head gas from the second column and - an intermediate fluid is sent from the second column to the third column. i) a first set of distillation columns comprising at least a first column operating at a first pressure, called high pressure, and a second column operating at a second pressure lower than the first pressure, the head of the first column being thermally connected to the bottom of the second column by means of a reboiler-condenser, a first purification unit and a first heat exchange line, and ii) a second set of distillation columns comprising at least a third column operating at a third pressure lower than the first pressure and a fourth column operating at a fourth pressure lower than the first, second and third pressures, the third column head being thermally connected to the bottom of the fourth column by a reboiler-condenser, a second purification unit (E') and a second exchange line heat, in which compressed air is sent to the first purification unit, the cleaned air is sent from the first purification unit to the first exchange line, cooled air is sent from the first heat exchange line to the first column, compressed air is sent to the second purification unit, cleaned air is sent from the second purification unit to the second line exchange, and cooled air is sent from the second exchange line to the third column operating at the third pressure characterized in that: - a head fluid is sent from the third column to the second column or the head gas from the third column is mixed with a head gas from the second column and - an intermediate fluid is sent from the second column to the third column. - a head fluid is sent from the third column to the second column or the head gas from the third column is mixed with a head gas from the second column and - an intermediate fluid is sent from the second column to the third column.