A foldable display device is designed so that a junction between at least two display panels connected to each other is not visibly seen. The precursor of functional nanocarbon produced on the catalyst and amorphous carbon secondarily produced on the catalyst react with the coexistent gas so that being removed from the catalyst, making it possible to prevent the drop of conversion with time on stream due to the inhibition of the reaction by the precursor and by-product. The sail cover has a first surface and a second surface that form a cavity therebetween. When the test circuitry is in the activated state, in some embodiments, the WL voltage andor the BL voltage applied to the SRAM cell may be different from the nominal voltages provided by the RWdecoder circuitry. As a consequence, the deformation of the lower-surface edge portions of the Si layer resulting from oxidation is less likely to occur and a leakage current resulting from a failure caused by the deformation of the lower-surface edge portions of the Si layer is suppressed.