A synchronous control method and apparatus are disclosed for a web rotary printing press which comprises a first printing press, a second printing press, a drive motor for driving the second printing press, and a folder provided in the first printing press, and enables a web printed by the first printing press and a web printed by the second printing press to be superposed and folded by the folder. The synchronous control apparatus comprises a pattern phase deviation detecting sensor provided halfway through a transport path, on which the web printed by the second printing press is transferred so that the web is superposed on a web printed by the first printing press, and adapted to measure the position of a pattern printed by the second printing press, and a pattern phase deviation computing device and a central control device for controlling the rotation phase of the drive motor based on the position of the pattern printed by the second printing press, the position having been measured by the sensor.