Other patients and their relatives fled for their lives in panic as multiple gunshots rang out through the room about 1:40pm.
Chaiyaporn Chaowanakamon, 31, was severely wounded but still alive, and was rushed into an operating room for emergency surgery.
Police said Chaiyaporn had accompanied his wife Thanyamas Thanamsri, 26, and her father Samphan Thanamsri, 60, to the hospital.
Ms Thanyamas, a tutor, had been suffering medical problems since giving birth six months ago and was there to see an obstetrician, according to media reports.
Chaiyaporn began to quarrel with Thanyamas's father, who did not like his son-in-law and wanted his daughter to end the relationship because her husband had no work and would not look for a job.
Chaiyaporn asked his wife to talk outside the hospital as there were many patients in the room waiting to see doctors. Samphan told his daughter not to go outside with him, the reports said.
The man became even more angry, opened his bag, pulled out a pistol and shot his father-in-law dead, and then his wife. He then turned the gun on himself.
Reports said Samphan was shot twice in the head and Thanyamas was shot in the chest, arms and back.
Two other people suffered minor injuries. They were identified as Duanglak Siripitrakul, 52, and Surawan Panthasing, 26.
The police investigation was continuing.
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