We have established tumor-resected mouse as a model animal to analyze the mechanism of depression lasting after cancer remission. Our previous findings demonstrated that tumor-resected mice show decreased sociability during at least 14 days after tumor resection. In the present study, we aimed to identify the specific cells, which are involved in abnormal sociability in tumor-resected mice. For preparation of tumor-resected mice, male BALB/c mice were intradermally inoculated 1×10⁷ cells of colon 26 into their abdomen, and surgically resected tumor formed 3 days after inoculation. Four days after tumor resection, we dissected the hippocampus, which was subjected to Western blotting and immunohistochemical analyses. It was seen in the hippocampus of tumor-resected mice that decreased expression level of Iba1, a microglial marker, and shortened process length of Iba1 positive cell. Tumor inoculation-induced abnormal sociability and shortened process of hippocampal Iba1 positive cells were attenuated by oral administration of fluoxetine (10 mg/kg) for 14 days from the first day after the treatment. These findings suggest that SSRI improves a long-lasting decrease in sociability with morphological changes in hippocampal microglial cells in tumor-resected mice.