To enable an operator to easily and rapidly grasp the occurrence of an abnormality in measurement performed by a sensor.

This monitor system is equipped with a CPU 33 which provides a display using first display aspects (43a, 43b and 43c) including a gauge section 41 acquiring a detection value, indicating the amount of fuel, from the sensor 31a for detecting the amount of fuel and serving as a criterion indicating the amount of fuel, and a needle image 41c presenting the amount of fuel corresponding to the detection value of the gauge portion 41, which determines whether or not the detection value normally indicating the amount of fuel is acquired, and which changes the display to a display using second display aspects (43a and 43c) different from the first display aspects, when determining the detection value normally indicating the amount of fuel is not acquired.