To prevent boilers from repeating combustion and stop in a short time even when occurrence and dissolution of communication abnormality occur frequently in multi-tubular boilers.

The multi-tubular boiler comprises a plurality of boilers 1 installed in parallel, and a unit number control device 3 outputting operation instructions to the plurality of boilers 1. The respective boilers are independently provided with boiler operation control devices 2. Each boiler operation control device 2 has a centralized control mode in which each boiler is operated based on the operation instructions from the unit number control device 3, and an individual control mode in which each boiler is operated independently. When the communication abnormality occurs, the individual control by the boiler operation control device 2 is performed. When the centralized control is switched to the individual control due to the occurrence of the communication abnormality, control is performed to continue the individual control until the lapse of a predetermined time even after the dissolution of the communication abnormality and to return to the centralized control after the lapse of the predetermined time from the dissolution of the communication abnormality.