An engine apparatus comprising: a cylinder block, a cylinder head, and a head cover; an intake manifold on a right side surface of the cylinder head and an exhaust manifold on a left side surface of the cylinder head; a blow-by gas returning device configured to return a leakage of blow-by gas from a combustion chamber to an intake system; a gas pressure regulation valve;an expansion chamber into which the blow-by gas is introduced through the gas pressure regulation valve, wherein lubricant contained in the blow-by gas is isolated in the expansion chamber and the blow-by gas is returned to an intake side of an engine from the expansion chamber, a lubricant return hole through which the lubricant isolated in the expansion chamber is returned to an inside of the engine, wherein in the expansion chamber, a plurality of maze conduits (129, 130, 131) are disposed, the plurality of maze conduits (129, 130, 131) being defined by a plurality of partition walls (126, 127, 128), characterized in that: the engine apparatus further comprises a blow-by gas exit adjacent to a center of a right-left width of a portion of the head cover where the expansion chamber is disposed so as to return the blow-by gas from the expansion chamber to the intake side of the engine through the blow-by gas exit, wherein the expansion chamber comprises a plate spring comprising a non-return valve openable and closable over a lower surface side opening of the lubricant return hole of the expansion chamber. a cylinder block, a cylinder head, and a head cover; an intake manifold on a right side surface of the cylinder head and an exhaust manifold on a left side surface of the cylinder head; a blow-by gas returning device configured to return a leakage of blow-by gas from a combustion chamber to an intake system; a gas pressure regulation valve;an expansion chamber into which the blow-by gas is introduced through the gas pressure regulation valve, wherein lubricant contained in the blow-by gas is isolated in the expansion chamber and the blow-by gas is returned to an intake side of an engine from the expansion chamber, a lubricant return hole through which the lubricant isolated in the expansion chamber is returned to an inside of the engine, wherein in the expansion chamber, a plurality of maze conduits (129, 130, 131) are disposed, the plurality of maze conduits (129, 130, 131) being defined by a plurality of partition walls (126, 127, 128), characterized in that: the engine apparatus further comprises a blow-by gas exit adjacent to a center of a right-left width of a portion of the head cover where the expansion chamber is disposed so as to return the blow-by gas from the expansion chamber to the intake side of the engine through the blow-by gas exit, wherein the expansion chamber comprises a plate spring comprising a non-return valve openable and closable over a lower surface side opening of the lubricant return hole of the expansion chamber. the engine apparatus further comprises a blow-by gas exit adjacent to a center of a right-left width of a portion of the head cover where the expansion chamber is disposed so as to return the blow-by gas from the expansion chamber to the intake side of the engine through the blow-by gas exit, wherein the expansion chamber comprises a plate spring comprising a non-return valve openable and closable over a lower surface side opening of the lubricant return hole of the expansion chamber.