A precombustion-chamber type gas engine, comprising: a main chamber defined between a piston and a cylinder head; a precombustion chamber which is in communication with the main chamber via an injection nozzle; a precombustion-chamber gas supply passage capable of supplying fuel gas to the precombustion chamber from a supply source of the fuel gas not via the main chamber; a check valve disposed in the precombustion-chamber gas supply passage; a supply pressure control valve which is disposed on an upstream side of the check valve in the precombustion-chamber gas supply passage and which is capable of adjusting a pressure (Pm, Ps) of the fuel gas to be supplied to the precombustion chamber, characterized by comprising a torch strength information acquisition device configured to obtain torch strength information correlated to strength of a torch from the injection nozzle, on the basis of a pressure in the main chamber and a pressure in the precombustion chamber; a precombustion-chamber gas supply amount calculation device configured to calculate an amount of the fuel gas to be supplied to the precombustion chamber from the precombustion-chamber gas supply passage (hereinafter, referred to as precombustion-chamber gas supply amount), on the basis of the torch strength information obtained by the torch strength information acquisition device and correlation information representing (Ir) a correlation between the torch strength information, a thermal efficiency, and the precombustion-chamber gas supply amount (Qs); and a precombustion-chamber gas supply pressure control device configured to control the supply pressure control valve on the basis of the precombustion-chamber gas supply amount calculated by the precombustion-chamber gas supply amount calculation device. a main chamber defined between a piston and a cylinder head; a precombustion chamber which is in communication with the main chamber via an injection nozzle; a precombustion-chamber gas supply passage capable of supplying fuel gas to the precombustion chamber from a supply source of the fuel gas not via the main chamber; a check valve disposed in the precombustion-chamber gas supply passage; a supply pressure control valve which is disposed on an upstream side of the check valve in the precombustion-chamber gas supply passage and which is capable of adjusting a pressure (Pm, Ps) of the fuel gas to be supplied to the precombustion chamber, characterized by comprising a torch strength information acquisition device configured to obtain torch strength information correlated to strength of a torch from the injection nozzle, on the basis of a pressure in the main chamber and a pressure in the precombustion chamber; a precombustion-chamber gas supply amount calculation device configured to calculate an amount of the fuel gas to be supplied to the precombustion chamber from the precombustion-chamber gas supply passage (hereinafter, referred to as precombustion-chamber gas supply amount), on the basis of the torch strength information obtained by the torch strength information acquisition device and correlation information representing (Ir) a correlation between the torch strength information, a thermal efficiency, and the precombustion-chamber gas supply amount (Qs); and a precombustion-chamber gas supply pressure control device configured to control the supply pressure control valve on the basis of the precombustion-chamber gas supply amount calculated by the precombustion-chamber gas supply amount calculation device.