A control valve comprising: a main flow passage; a main flow passage sectional area adjustment spool mounted midway in the main flow passage in order to increase and reduce a flow passage sectional area; a downstream side branch flow passage that bifurcates from the main flow passage on a downstream side of a position of the flow passage sectional area adjustment spool; a communicating flow passage for delivering oil from the downstream side branch flow passage toward the flow passage sectional area adjustment spool; a flow passage opening/closing valve mounted between the downstream side branch flow passage and the communicating flow passage in order to connect and cut off the downstream side branch flow passage and the communicating flow passage to and from each other; a communicating flow passage opening/closing spool mounted midway in the communicating flow passage; and an upstream side branch flow passage that bifurcates from the main flow passage on an upstream side of the position of the flow passage sectional area adjustment spool in order to supply an oil pressure to the flow passage opening/closing spool, wherein, in a low rotation region of an engine, the flow passage opening/closing valve cuts off the downstream side branch flow passage from the communicating flow passage so that the flow passage sectional area of the main flow passage is maximized, in a medium rotation region of the engine, the flow passage opening/closing valve connects the downstream side branch flow passage to the communicating flow passage and the flow passage opening/closing spool allows the communicating flow passage to communicate such that the flow passage sectional area adjustment spool is moved by the oil pressure in a direction for reducing the flow passage sectional area of the main flow passage, and in a high rotation region of the engine, the flow passage opening/closing spool blocks the communicating flow passage such that the flow passage sectional area of the main flow passage is maximized. a main flow passage; a main flow passage sectional area adjustment spool mounted midway in the main flow passage in order to increase and reduce a flow passage sectional area; a downstream side branch flow passage that bifurcates from the main flow passage on a downstream side of a position of the flow passage sectional area adjustment spool; a communicating flow passage for delivering oil from the downstream side branch flow passage toward the flow passage sectional area adjustment spool; a flow passage opening/closing valve mounted between the downstream side branch flow passage and the communicating flow passage in order to connect and cut off the downstream side branch flow passage and the communicating flow passage to and from each other; a communicating flow passage opening/closing spool mounted midway in the communicating flow passage; and an upstream side branch flow passage that bifurcates from the main flow passage on an upstream side of the position of the flow passage sectional area adjustment spool in order to supply an oil pressure to the flow passage opening/closing spool, wherein, in a low rotation region of an engine, the flow passage opening/closing valve cuts off the downstream side branch flow passage from the communicating flow passage so that the flow passage sectional area of the main flow passage is maximized, in a medium rotation region of the engine, the flow passage opening/closing valve connects the downstream side branch flow passage to the communicating flow passage and the flow passage opening/closing spool allows the communicating flow passage to communicate such that the flow passage sectional area adjustment spool is moved by the oil pressure in a direction for reducing the flow passage sectional area of the main flow passage, and in a high rotation region of the engine, the flow passage opening/closing spool blocks the communicating flow passage such that the flow passage sectional area of the main flow passage is maximized.