A pipe coupling structure includes an electromagnetic valve and a passage block having an upper surface mounted with the electromagnetic valve and opposite side surfaces formed with an inlet port and an outlet port. The inlet port is connected to a PCV pipe through an inlet pipe joint, and the outlet port is connected to a PCV pipe through an outlet pipe joint. At least one of the inlet pipe joint and the outlet pipe joint is a union joint. The union joint includes a hollow cylindrical union end making contact with a first surface formed with the inlet port or a second surface formed with the outlet port and a hollow cylindrical union nut configured to connect the inlet port or the outlet port to the union end.
