This paper presents a construction of a flip-flop (FF) that works as a soft error tolerant FF in system operations and as a BILBO (Built-In Logic Block Observer) 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. This paper also shows an evaluation result indicating the area of the proposed FF is 11.4% 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 34.8% shorter than that for the combination.