A precombustion-chamber type gas engine, comprising includes: a check valve disposed in the precombustion-chamber gas supply passage and configured to block a backflow of fuel gas from a precombustion chamber; 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 of the fuel gas to be supplied to the precombustion chamber; 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 a precombustion-chamber gas supply amount, on the basis of the torch strength information and correlation information representing a correlation between the torch strength information, a thermal efficiency, and the precombustion-chamber gas supply amount; 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.