There are provided a synthetic resin joined body and a method of manufacturing the same, with which it is possible to secure both of sealing performance of a fluid passage and joint strength between synthetic resin parts, even if an area of a portion to be welded is reduced. The synthetic resin joined body includes: a baffle plate; an oil passage forming member which comes in contact with the baffle plate; an oil passage which is formed between the baffle plate and the oil passage forming member and through which lubricant flows; a sealing portion which is formed by welding the baffle plate and the oil passage forming member to each other on an outer side of the oil passage to seal a periphery of the oil passage; and a joint portion for joining the baffle plate and the oil passage forming member to each other.