To accurately measure a retardation of a resin film in a production line by regulating a difference between a maximum value and a minimum value in a width direction distribution of a tension operating in a running direction of the film to a specific value or less.

The system 10 for measuring a retardation comprises a tension regulator 12, a retardation value measuring unit 14, and a rail 16 and guide rolls 18 for moving the unit 14. The regulator 12 makes a distribution of a tension operating in an advancing direction of a resin film 20 slackened in a width direction constant. That is, a difference between a maximum value and a minimum value of the distribution of the tension operating in the advancing direction to the film 20 in a width direction is regulated to 4 kg/m or less, preferably 2 kg/m or less or further preferably 1 kg/m or less. Thus, a measured result equivalent to that measured in a no load state for the retardation of the retardation film in a production line is obtained. Thus, a condition regulation in the production is facilitated.