It usually takes several days to detect slow-growing bacteria by a blood culture system. Moreover, even if the microbial growths are detected in blood samples, they will escape our notice at a microscopic examination using Gram’s stain. Consequently, the results are often regarded as false positive.　　This paper reports case of malignant lymphoma in which bloodstream infection caused by slow-growing spirillum was observed. The patient in chemotherapy for malignant lymphoma, complained of repeated fever. Repeated blood cultures were taken and occasionally positive signals, which mean increased CO2 concentrations in the culture bottle, were detected by BACTEC 9240 (Becton Dickinson). However, routine microscopic examination with Gram’s stain did not detect any bacteria. Thus, the results of BACTEC were thought to be false positive. Thereafter, the bacterial culture period was extended. Finally, a spirillum, suspected of one of Helicobacter species, was observed microscopically. The detected spirillum was regarded as Helicobacter canadensis with 98.08% homology, using polymerase chain reaction with the 16S rRNA method and basic local alignment search tool (BLAST).　　H. canadensis is one of new species isolated from humans with diarrhea. This bacterium is considered to cause a zoonotic infection. There have been some case reports that this bacterium infected immunosuppressive patients, so we should exercise caution against such conditions. Moreover, we should keep vigilant against the spread of slow-growing bacteria when there are discrepancies in findings between blood culture system and microscopic examination. It is useful to extend the culture period to detect such slow-growing bacteria.