To provide a selection system for multiplex transmission lines where diagnosis execution is hardly disabled and a degree of freedom of avoiding system-down can be enhanced.

A 1st main controller 2 comprises a 1st main control function section 8 and a 1st main system selection function section 9, and a 2nd main controller 5 comprises a 2nd main control function section 11 and a 2nd main system selection function section 12. The confounding transmission line comprises a 1st confounding transmission line 19 that connects the 1st main control function section 8 to a subordinate device 3 via the 2nd main system selection function section 12 and a 2nd confounding transmission line 16 that connects the 2nd main control function section 11 to a 2nd subordinate device 6 via the 1st main system selection function section 9. The 1st confounding transmission line 19 is activated on the basis of deactivation between the 1st main controller 2 and the 1st subordinate device 3, and the 2nd confounding transmission line 16 is activated on the basis of deactivation between the 2nd main controller 5 and the 2nd subordinate device 6.