My hyunadai sonata 2004 model, losing fuel pressure when I stop its engine. No visible leak from injectors and fuel pressure regulator. No fuel leak through vacuum tube connected to fuel pressure regulator. Please give me a solution. My engine is cranking to start after a long parking

In most cases, leaking injectors cannot be "seen" because the fuel is dripping inside the intake manifold runners and into the engine, which results in an extremely rich mixture, thus requiring extended cranking to purge some of the raw fuel before the engine can start.
Extended cranking can also be caused by the fuel pump leaking pressure directly back into the fuel tank. Again, very difficult to detect.
If there is a place on the fuel lines where you can pinch it off, try getting the fuel rail charged and then pinching off the supply and return (if equipped) before cycling the key off. That will hold full pressure on the injectors while separating the fuel pump from the pressure gauge. Pressure should hold on the injectors if the fuel pump is the culprit. If the fuel pressure begins to drop evenly, release the clamp on the supply side to see if the "back half" can recharge the rail. Clear as mud yet? Let me know...

Modern day fuel systems tend to be very reliable. You may need to replace the Engine Coolant Temperature (ECT) Sensor, which affects fuel injector pulse width. The ECT Sensor is a temperature dependent resistor (thermistor), whose resistance decreases as the engine coolant temperature increases. It provides a richer mixture when cold, leaner mixture when hot. They become corroded, due to not changing the coolant frequently enough, flood the engine, and waste a tremendous amount of fuel. Are your spark plugs fuel soaked?