We report the rare case of a 59-year-old man with motor neuron disease and semantic dementia (SD-MND); SD-MND was in a very early stage, and its clinical progression, especially with regard to language impairment, and abnormalities on neuroimages were evaluated for 3 years. The patient complained only of difficulties in recalling names of acquaintances and in writing kanji characters. After 1 year, he experienced difficulty in describing common objects. He developed two-way anomia only in some words, which varied from day to day. His anomia was not category-specific and was noted even with respect to words that describe color. In addition to experiencing difficulty in writing kanji characters, he experienced difficulty in writing kana characters. Muscle atrophy was observed, and he experienced weakness in his limbs, especially in the right upper limb; however, bulbar symptoms were not observed. At this point, he fulfilled the diagnostic criteria for MND. In the next year, semantic memory impairment became apparent, and he was subsequently diagnosed with SD. Deterioration in his ability to name objects in all categories, except body parts, was noted. Further, the ability of writing both kana and kanji characters was increasingly impaired. He developed bulbar symptoms and experienced increased muscle weakness. The characteristics of this patient differed from those of SD patients without MND with regard to the difficulty in writing kana characters and naming colors even though the SD-MND was in the early stage. Further, the pattern of brain hypoperfusion was different from that observed for SD patients without MND. In the case of this patient, brain hypoperfusion was found not only in the left anterior temporal lobe but also in the frontal lobe. The characteristics of his language symptoms might be related to the specific pattern of brain hypoperfusion, which might be commonly observed in patients with dementia and MND.