Tumor spheroids are now intensively investigated toward preclinical and clinical applications, necessitating the establishment of accessible and cost-effective methods for routine operations. Without losing the advantage of organ-chip technologies, we developed a rocking system for facile formation and culture of tumor spheroids in hydrogel microwells of a suspended membrane under microfluidic conditions. While the rocking is controlled with a step motor, the microfluidic device is made of two plastic plates, allowing plugging directly syringe tubes with Luer connectors. Upon injection of the culture medium into the tubes and subsequent rocking of the chip, the medium flows back and forth in the channel underneath the membrane, ensuring a diffusion-based culture. Our results showed that such a rocking- and diffusion-based culture method significantly improved the quality of the tumor spheroids when compared to the static culture, particularly in terms of growth rate, roundness, junction formation and compactness of the spheroids. Notably, dynamically cultured tumor spheroids showed increased drug resistance, suggesting alternative assay conditions. Overall, the present method is pumpless, connectionless, and user-friendly, thereby facilitating the advancement of tumor-spheroid-based applications.