The limited mouth opening, also known as trismus, can result from temporomandibular joint disorders, infection, neoplasm, trauma, and abnormal anatomic structure like coronoid hyperplasia. Head and neck cancer patients often complain of limited mouth opening, which is usually induced by myofibrotic contracture of masticatory muscle. But clinicians should consider any reasons such as infection or cancer growth and metastasis if trismus gets worse or pain develops. We report the case of the patient, who was diagnosed with myxofibrosarcoma on nasal cavity. He had suffered from trismus after concurrent chemoradiotherapy. However, pain had developed and trismus had worsened. He was diagnosed with infratemporal space abscess and myositis of masticatory muscles.