The present invention relates to an appearing and disappearing core type writing instrument capable of appearing and disappearing a core based on a rotation operation. The present invention relates to a rubber composition for a sidewall, an insulation, or a breaker cushion, including: a rubber component that contains a modified natural rubber with a phosphorus content of 200 ppm or less; and carbon black andor a white filler. When an optical signal is transmitted, light transmission is limited by increasing a refractive index difference between positions with different optical field intensity in the core layer, thus reducing a loss of optical signal leakage and improving uplink transmission efficiency. The resulting dieboard is relatively free of surface defects.