© 2018 The Japan Society of Applied Physics. We demonstrated 20-Gbit/s direct modulation with a low energy cost of the membrane distributed-reflector (DR) laser whose static lasing characteristics were previously reported by Tomiyasu et al. in Appl. Phy. Express 10, 062702 (2017). As a small-signal modulation characteristic, a modulation current efficiency of 12 GHz/mA1/2was obtained. In 20-Gbit/s direct modulation using a non-return-to-zero 211- 1 word-length pseudorandom bit sequence signal, a clear eye opening with a bit-error rate of 6.4 × 10-10was obtained with a bias current of 1.06 mA and a bias voltage of 1.76 V, which resulted in a record low energy cost of 93 fJ/bit among membrane distributed-feedback (DFB) and DR lasers ever reported.