We report a pediatric case of critical illness polyneuropathy and myopathy caused by Bacillus cereus sepsis during acute lymphoblastic leukemia therapy. A 15-year-old boy developed B. cereus sepsis and multiple organ failure on the 19th day after initiation of chemotherapy, and multidisciplinary treatment was started. Treatment was effective and septic shock with multiple organ failure remitted. He was weaned from a respirator on day 23 after the onset of sepsis, but complete flaccid paralysis of the 4 extremities occurred. His compound muscle action potential and F-wave occurrence were reduced on a nerve conduction test. The number of motor units was markedly decreased, and the amplitude and duration of individual motor units were low and short, respectively, on electromyography. Cerebrospinal fluid was normal. On the basis of these findings, he was diagnosed with critical illness polyneuropathy/myopathy. He underwent intensive rehabilitation and recovered the ability to walk 3 months after onset. He was discharged 1 year after the initiation of chemotherapy, and remission has been maintained without inconvenience to daily living activities for 3 years since disease onset.