Where's the rest of the details? Does the pressure come up when you raise engine speed? Does the "Check Gauges" light turn on? How low is the gauge dropping to?
If there are no other symptoms, the most common cause is a defective sending unit. If the light also turns on, the actual pressure should be checked with a mechanical pressure gauge. If it really is low, the most common cause is worn crankshaft and connecting rod bearings. You may help the situation for a while by using a higher viscosity oil.