We describe a case of primary mucinous (colloid) adenocarcinoma of the lung. The patient was a 61-year-old male with a peripheral mass of the left lung. Retrospectively, the mass, which showed slow growth later, could be seen in a chest X-ray 3 years before surgery. The tumor was filled with a gelatinous substance resembling strawberry jelly, macroscopically. Microscopically, it was mucin with a small amount of malignant cells (adenocarcinoma). Diagnosis was mucinous (colloid) adenocarcinoma, which is a very rare intrapulmonary neoplasm. Mucinous (colloid) adenocarcinoma is suspected when a gelatinous substance like strawberry jelly is observed intraoperatively in the lung.