We report two cases of lung cancer demonstrating unilateral shoulder pain as a result of diaphragm invasion. Case 1: A 66-year-old male noticed left shoulder pain three months prior to his seeking medical treatment and was found to have a mass shadow at the bottom of the left lower lobe on chest X-ray. His diagnosis was squamous cell carcinoma of the lung invading the left diaphragm. Case 2: A 55-year-old female suffered from cough for two weeks and noticed right shoulder pain at the time of her consultation. Chest X-ray revealed mass shadow on the bottom of the right lower lobe, which invaded the right diaphragm on chest CT scan, and the diagnosis was lung cancer. In both cases, chemotherapy was ineffective, and the shoulder pain deteriorated as the tumor grew in size. The mechanism of referred shoulder pain is thought to be possession of the common root of the phrenic nerve's sensory branch with the C3 C5 sensory nerve corresponding to the shoulder area. Diseases of the diaphragm should be taken into consideration in shoulder pain without orthopedic disturbances.