A scroll compressor is provided that may include a differential pressure hole formed at or in an orbiting scroll that communicates a high pressure part with an intermediate pressure part, a decompression portion formed in the differential pressure hole and having a pin member inserted therein to decompress oil. Preferably the following relationships are satisfied: \xbd Wg\u2266Wpg<Wg; and Wpg<Wp, where Wp, Wpg and Wg represent a width of the pre-pit, the width of the groove at the junction parts, and the width of the other part of the groove other than the junction parts, respectively. Portions of the B-mode image signal associated with stationary tissue are intact while portions of the B-mode image signal associated with flow are substantially suppressed. The pair of shielding plates 17 and 18 divides the inside of the outer cylindrical casing 4 in front and rear and is provided with one or more air holes 15 and 16 at the outer circumferential section or at the central section.