CHOP-like regimen combined with rituximab is a standard chemotherapy for diffuse large B-cell lymphoma (DLBCL). The relative dose intensity (RDI) was proposed as an index of the dose and administration interval of agents. Previous studies reported that the maintenance of the RDI during CHOP therapy improved the treatment results. However, few studies regarding RDI have reviewed patients receiving combination therapy with CHOP and rituximab. We investigated the influence of RDI maintenance, involving combination therapy with rituximab, on therapeutic effects in patients with DLBCL. We retrospectively examined 152 DLBCL patients who were treated with CHOP-like regimen combined with rituximab in whom the RDI could be followed up. Multivariate analysis revealed that international prognosis index (IPI) high intermediate-high (HI-H) (p = 0.005) and RDI of less than 70% (p = 0.007) were independent prognostic factors for low progression free survival. Concerning overall survival, IPI HI-H (p = 0.027) and an RDI of less than 70% (p = 0.002) were involved in an unfavorable prognosis. In addition, age over 60 years (p = 0.003), R-THPCOP (p = 0.034), or the presence of febrile neutropenia (p = 0.004) made RDI maintenance difficult, and prophylactic G-CSF therapy (p = 0.026) was useful for maintaining the RDI. Maintaining the RDI is important even in the era of rituximab-combined chemotherapy for DLBCL.