Sleep apnea syndrome（SAS）is characterized by apnea and hypopnea during sleep. SAS manifests various symptoms, and can become a risk factor for a variety of diseases. Typical psychiatric presentations of SAS are depressive symptoms, and those resembling negative symptoms in schizophrenia.
 We report two patients with schizophrenia spectrum disorders. Both patients showed the partial improvement of psychiatric symptoms with pharmacotherapy. After diagnosing comorbid SAS and subsequent treatment with continuous positive airway pressure（CPAP）, the psychiatric symptoms improved.
 The first case was a 54‒year‒old woman, who presented with auditory hallucinations and delusions and was diagnosed with schizophrenia at 32 years of age. Her positive symptoms responded immediately to medication；however, her negative symptoms persisted despite switching to atypical antipsychotics. We diagnosed her with SAS using pulse oximetry and portable polysomnography（PSG）, and, after treatment with CPAP, her fatigue and shallow sleep improved, as well as her quality of life（QOL）.
 The second case was is a 61‒year‒old man, who presented with delusions of persecution and was diagnosed with delusional disorder at 49 years of age. His delusional symptoms fluctuated under medication, and repeatedly worsened under stressful situations. We suspected SAS as a Complicating factor, and diagnosed him with severe SAS using PSG. After treatment with CPAP, his hypertension and delusions of persecution improved.
 Screening for SAS is available in psychiatric hospitals and outpatient clinics. We believe that the possibility of comorbid SAS in psychiatric patients should be more widely acknowleged in clinical psychiatry.