We report a rare case of pulmonary enteric adenocarcinoma (PEA) exhibiting a immunohistochemical feature of CK7/CK20 double-negativity by evaluating the transformation zone between PEA and conventional pulmonary adenocarcinoma (CPA). A 75-year-old man was found to have a mass, 40 mm in diameter, in the right lower lobe on chest computed tomography, and underwent right lower lobectomy. Histologically, the tumor was composed of a PEA and CPA component. The dominant PEA component had medium to large complex glands with tall columnar cells with eosinophilic cytoplasm and brush-border. The CPA component comprised small to medium glands with cuboidal cells. Moreover, intermediate glands (INT), which had cuboidal to tall columnar cells, with morphological features between PEA and CPA, was also observed in the transformation area. Immunohistochemically, the PEA component was negative for CK7, CK20, and TTF-1, and positive for CDX2 and SATB2 (weak): the CPA component was negative for CK20, CDX2, and SATB2, and positive for CK7 and TTF-1: the INT were negative for SATB2, with intermingled positive signals for CK7, CK20, TTF-1, and CDX2. The final diagnosis was PEA based on the CPA component and not colorectal carcinoma. To distinguish CK7-negative PEA from metastatic colorectal carcinoma, careful examination for a CPA component is very useful along with clinical information. There are no reports that discuss about process of oncogenesis, de novo sequence or transformation from CPA of PEA. This is the first reported case of CK7/CK20 double-negative PEA, with analysis of the transformation zone between PEA and CPA components.