To provide an apparatus capable of measuring online during printing the expansion/contraction amount of a printing sheet printed using an offset rotary press, and an expansion/contraction preventing method for the printing sheet using the device.

The expansion/contraction amount measuring apparatus for a printing sheet printed with an offset rotary press is provided with a first sensor for measuring the width direction size of a printed printing sheet (or printing sheet) on the downstream side of a printing section in the offset rotary press, a second sensor for measuring the width direction size of the dried printing sheet on the downstream side in the press, a computing unit for inputting respective sizes measured by the first and the second sensors to compute the expansion/contraction amount by the size difference, and a display apparatus displaying computed data. The sensor for measuring the size of the width direction of the printing sheet is a sensor selected from the sensors employed by an image sensor and laser size measuring instrument.