A 57-year-old man with amyotrophic lateral sclerosis (ALS) was admitted because of depressive state. Selective serotonin reuptake inhibitor (SSRI), an antidepressant, was started on the admission day. The throbbing headache in the right temporal region appeared on day 3, and an analgesic drug was not completely effective. Serum sodium value on admission was 131 mEq/l. After SSRI was started the hyponatremia rapidly progressed, and it became 112 mEq/l on day 4. SSRI was discontinued, and headache disappeared; serum sodium was improved to 129 mEq/l. It has been reported that syndrome of inappropriate secretion of antidiuretic hormone (SIADH) may be associated with ALS. We consider that subclinical SIADH was manifested by SSRI in this case. SSRI seems to be a cause of headache, since headache disappeared completely by discontinuation of SSRI.