Multifocal motor neuropathy (MMN) is an uncommon, asymmetric motor neuropathy. As MMN is a treatable disorder, its differentiation from lower motor neuron disease is important. Evidence of conduction block (CB) or positive IgM anti-GM1 is considered one of important markers for the diagnosis. However, some patients with atypical MMN have no detectable CB or anti-GM1 antibody. We experienced a case of MMN with focal nerve enlargement on ultrasound. Ultrasound can be a valuable tool in supporting the diagnosis of MMN.