Bunyaviruses, including the Lassa virus (LASV), are known to cause hemorrhagic fever and have a high fatality rate among hospitalized patients, as there are few effective treatments. We focused on the fact that bunyaviruses use cap-dependent endonuclease (CEN) for viral replication, which is similar to influenza viruses. This led us to screen carbamoyl pyridone bicycle (CAB) compounds, which compose a series of baloxavir acid (BXA) derivatives, against lymphocytic choriomeningitis virus (LCMV) and Junin virus (JUNV) among the bunyaviruses. This led to the discovery of 1c, which has potent anti-bunyaviral activities. In SAR studies, we found that a large lipophilic side chain is preferred for the 1-position of the CAB scaffold, similar to the influenza CEN inhibitor, and that a small alkyl group for the 3-position shows high activity. Moreover, the 7‑carboxyl group of the scaffold is essential for anti-bunyaviral activities, and the antiviral activity is reduced by conversion to various carboxylic acid bioisosteres. The SAR results are discussed using a binding model of 9d in the active center of the known LCMV CEN crystal structure. These compounds show promise as broad-spectrum anti-bunyavirus therapeutics, given their relatively favorable metabolic stability and PK profiles.