A gas engine system comprising: a gas engine; a gas passage for supplying a fuel gas to the gas engine; a gas valve for opening and closing the gas passage; a control device for recognizing opening and closing of the gas valve, and controlling the gas valve; a pressure sensor for outputting pressure information corresponding to a gas pressure in the gas passage on a downstream side of the gas valve; and a transfer mechanism for transmitting the pressure information from the pressure sensor to the control device, wherein the control device comprises: a first determination section for determining, as a first determination, whether or not the gas pressure is a low pressure of less than a predetermined pressure on the basis of the pressure information, while the control device recognizes that the gas valve is closed; an opening section for opening the gas valve if it is determined that the gas pressure is the low pressure in the first determination; a second determination section for determining, as a second determination, whether or not the gas pressure is a high pressure of equal to or more than the predetermined pressure on the basis of the pressure information, while the control device recognizes that the gas valve is open, if it is determined that the gas pressure is the low pressure in the first determination; a closing section for closing the gas valve after the second determination is performed; and an abnormality determination section, characterised in that the system is configured to perform start-up control, wherein: the abnormality determination section is configured to perform abnormality detection control comprising: determining if a first abnormality has occurred, if it is determined that the gas pressure is the high pressure in the first determination; and determining if a second abnormality has occurred, if it is determined that the gas pressure is the low pressure in the second determination. a gas engine; a gas passage for supplying a fuel gas to the gas engine; a gas valve for opening and closing the gas passage; a control device for recognizing opening and closing of the gas valve, and controlling the gas valve; a pressure sensor for outputting pressure information corresponding to a gas pressure in the gas passage on a downstream side of the gas valve; and a transfer mechanism for transmitting the pressure information from the pressure sensor to the control device, a first determination section for determining, as a first determination, whether or not the gas pressure is a low pressure of less than a predetermined pressure on the basis of the pressure information, while the control device recognizes that the gas valve is closed; an opening section for opening the gas valve if it is determined that the gas pressure is the low pressure in the first determination; a second determination section for determining, as a second determination, whether or not the gas pressure is a high pressure of equal to or more than the predetermined pressure on the basis of the pressure information, while the control device recognizes that the gas valve is open, if it is determined that the gas pressure is the low pressure in the first determination; a closing section for closing the gas valve after the second determination is performed; and an abnormality determination section, determining if a first abnormality has occurred, if it is determined that the gas pressure is the high pressure in the first determination; and determining if a second abnormality has occurred, if it is determined that the gas pressure is the low pressure in the second determination.