In this paper, a soft-error-tolerant BILBO (Built-In Logic Block Observer) FF (flip-flop) is presented. The proposed FF works as a soft-error-tolerant FF in system operations and as a BILBO FF in manufacturing testing. The construction of the proposed FF is based on that of an existing soft-error-tolerant FF, namely a BISER (Built-In Soft Error Resilience) FF. The proposed FF contains a reconfigurable C-element with XNOR calculation capability, which works as a C-element for soft-error-tolerance during system operations and as an XNOR gate employed in linear feedback shift registers (LFSRs) during manufacturing testing. The evaluation results shown in this paper indicate that the area of the proposed FF is 8.5% smaller than that of a simple combination of the existing BISER and BILBO FFs. In addition, the sum of CLK-Q delay and D-CLK setup times on system operations for the proposed FF is 19.7% shorter than that for the combination.