Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by the selective death of motor neurons. As a mechanism of motor neurodegeneration, neuroinflammation by immune cells such as glial cells and other peripheral immune cells infiltrating in the central nervous system has been suggested. However, it is still unclear how the immune cell activation is regulated. We focused on immune checkpoint molecules (e.g. PD-1 and LAG-3) and analyzed their expression in ALS model mice (SOD1^G93A mice). After the disease onset, when compared to WT mice, SOD1^G93A mice showed an increased expression of membrane and soluble forms of LAG-3 in the spinal cord, but no change was detected in LAG-3 expression in peripheral tissues such as spleen and lymph nodes. We performed immunohistochemical analyses in the spinal cord of SOD1^G93A mice and found that LAG-3 was expressed in microglia. Using the mouse microglial cell line BV2, we analyzed changes in LAG-3 expression upon differentiation into inflammatory (M1)/anti-inflammatory (M2) state. We found that the LAG-3 mRNA expression levels were upregulated in M1 microglia, but not in M2 microglia. Elucidation of LAG-3 function in microglia will lead to a better understanding of the involvement of the immune checkpoint molecule in the pathophysiology of ALS.