A micro-pilot injection ignition type gas engine that ignites and burns premixed air fuel-gas mixture inducted into each main combustion chamber, by means of at least one pilot flame that jets out from a pre-chamber to the main chamber through at least one pre-chamber nozzle hole communicating the main chamber to the pre-chamber in which a fuel-oil injector injects micro-pilot fuel oil to produce a pre-chamber combustion gas as a source of the flame jet, the gas engine comprising: a gas valve that delivers and cuts a fuel-gas at a position in front of each cylinder, so as to arbitrarily control the throat area as well as an opening-or-closing time span of the gas valve; an engine speed detecting unit to detect the engine speed; a cylinder pressure sensor provided at each cylinder so as to continually measure pressure levels as to each cylinder; a combustion diagnosis unit to monitor the pressure levels so that the diagnosis unit analyzes an engine combustion state by means of a cylinder pressure distribution as a function of elapsed time, the distribution as to each cylinder being formed in the diagnosis unit; an opening-closing control unit as to the gas valve, adapted to control an intermittent opening-or-closing of the gas valve in response to the engine speed and the pressure distribution, wherein in starting the engine, at least one skip-firing mode that brings an enhanced fuel-supply pressure-pulsation with which an increased amount of fuel-gas is supplied per one engine-cycle with firing is performed so that the air fuel-gas ratio of each cylinder reaches a prescribed target value, wherein the opening-closing control unit as to the gas valve makes the gas valve for each cylinder open and close, intermittently per each cylinder in turn according to a firing sequence of the engine with the at least one skip-firing mode, so that the air fuel-gas ratio reaches the prescribed target value, and with a series of time spans as to the variable valve-opening-time span of the gas valve, the time spans being changed step by step and idling being omitted, and wherein the opening-closing control unit makes the gas valve for each cylinder open and close with a first skip-firing mode of a lower valve-opening-ratio in the case when the cylinder pressure distribution as a function of elapsed time the distribution which is analyzed by the combustion diagnosis unit does not reach a prescribed target distribution, whereas the opening-closing control unit makes the gas valve open and close with a second skip-firing mode of a higher valve-opening-ratio in the case when the cylinder pressure distribution as a function of elapsed time corresponds to the prescribed target distribution. a gas valve that delivers and cuts a fuel-gas at a position in front of each cylinder, so as to arbitrarily control the throat area as well as an opening-or-closing time span of the gas valve; an engine speed detecting unit to detect the engine speed; a cylinder pressure sensor provided at each cylinder so as to continually measure pressure levels as to each cylinder; a combustion diagnosis unit to monitor the pressure levels so that the diagnosis unit analyzes an engine combustion state by means of a cylinder pressure distribution as a function of elapsed time, the distribution as to each cylinder being formed in the diagnosis unit; an opening-closing control unit as to the gas valve, adapted to control an intermittent opening-or-closing of the gas valve in response to the engine speed and the pressure distribution, wherein in starting the engine, at least one skip-firing mode that brings an enhanced fuel-supply pressure-pulsation with which an increased amount of fuel-gas is supplied per one engine-cycle with firing is performed so that the air fuel-gas ratio of each cylinder reaches a prescribed target value, wherein the opening-closing control unit as to the gas valve makes the gas valve for each cylinder open and close, intermittently per each cylinder in turn according to a firing sequence of the engine with the at least one skip-firing mode, so that the air fuel-gas ratio reaches the prescribed target value, and with a series of time spans as to the variable valve-opening-time span of the gas valve, the time spans being changed step by step and idling being omitted, and wherein the opening-closing control unit makes the gas valve for each cylinder open and close with a first skip-firing mode of a lower valve-opening-ratio in the case when the cylinder pressure distribution as a function of elapsed time the distribution which is analyzed by the combustion diagnosis unit does not reach a prescribed target distribution, whereas the opening-closing control unit makes the gas valve open and close with a second skip-firing mode of a higher valve-opening-ratio in the case when the cylinder pressure distribution as a function of elapsed time corresponds to the prescribed target distribution.