A device detects the cam top position in an in-line type high pressure pump with a decreased amount of time required to detect the rotation position of the cam top position in an engine starting operation, thereby decreasing the time for starting an engine. A cam for driving a plunger has a plurality of cam lobes, and a plurality of cam top position pointers are attached to the outer periphery of the camshaft or to the outer periphery of a flywheel connected to the camshaft such that each of the pointers corresponds to each of the cam lobes of the cam in number and angle phase, respectively. A cam top position detector is located to face the pointers when the camshaft rotates so that the pointers are detected by the detector.