[Introduction] Painless thyroiditis is characterized by transient hyperthyroidism, followed by hypothyroidism, and spontaneous recovery. The pathogenesis of painless thyroiditis is considered a variant form of chronic autoimmune thyroiditis. It is thought to be part of the spectrum of thyroid autoimmune disease, however many parts of the pathogenisesis are still unknown. Not so many reports were shown about perioperative painless thyroiditis. A case with painless thyroiditis developped after laparoscopic surgery was presented. [Patient] A 23 year old Japanese woman P0G0 was referred to our department pointed out ovarian tumor by lower abdominal CT test. She visited the hospital with lower abdominal pain. Cystic teratoma was diagnosed by Image study. Laparoscopic timorectomy was performed and hitological report shouwed cystic teratoma (operation time 104 min. Blood loss 10g). After operation, tachycardia (120 -140bpm) and palpitation were observed. Blood examination showed TSH 0.004 IU/ml, free T4 level 7.96 ng/dl, suggesting thyrointoxication. No tenderness or swelling of thyroid was found. Blood examination about antibodies showed that thyroglobulin antibody was positive and TSH receptor antibody level was with in normal range. Therefore, thyrointoxication caused by painless thyroiditis was dignosed. By teating beta blocker as the symptomatic treatment for one month, any symptoms were no longer observed. [Conclusions] Thyroid disorder of this case was thought because of painless thyroiditis developed from chronic autoimmune thyroiditis. The cause for this development was unclear, however, operative stress might cause this disorder. The painless thyroiditis might be considered in the differential diagnosis of the post operative case with tachycardia or palpitation by unknown cause.