The explosion occurred midday at a four-storey commercial building in Soi Rang Nam near the Victory Monument and Sri Ayudthya School when a mechanic was trying to fix the valve of a 48-kilogramme gas cylinder.
The process to open the valve caused leakage of flammable gas and led to the explosion which completely destroyed the ground floor of the steak house with shattered glass.
The blast also injured five people including staff and customers. One person was severely injured while four others suffered with minor injuries caused by broken glass.
The other casualties were transferred to a nearby hospital for medical treatment.
Meanwhile, police investigators of Phaya Thai police station initially presumed that the explosion was an error, occurring when gas valve was opened. An investigation is underway and police have not filed charges against anyone.
Ratchathewi district authorities reopened the road and footpath in front of the restaurant after the flames were brought under control and the situation returned to normal.
Ratchathewi district officer Laksan Rojthamrong said after inspecting the scene that the fire, which did not spread to the nearby buildings, would not impact the structure of the building overall and that there was no need to declare the area off-limits