PURPOSE: To reduce number of wirings between a controller body and active elements, various sensors and to delete a cost of the wirings themselves and a noise remedy cost in a controller for a boiler which judges an operating state of the boiler according to inputs from the sensors and outputs a designation signal for starting/stopping a boiler accessory unit, switching a a solenoid valve via the elements such as relay, in response to the state.

CONSTITUTION: The controller for a boiler comprises a serial/parallel converting mechanism 12 for transmitting information of designating operations of active elements 24 from a controller body 10 aligned as a serial signal, a serial/parallel converting mechanism 21 for sorting information of serial signal and transmitting it to the elements 24, a parallel/serial converting mechanism 22 for transmitting detection signals from sensors aligned as a serial signal, and a serial/ parallel converting mechanism 13 for sorting information of its serial signal and individually transmitting to the body 10.