A monitoring device for an engine control unit, the monitoring device configured to monitor, by comparing a required fuel injection amount determined according to an operation state of an internal combustion engine and a monitored injection amount calculated using an injection monitoring signal generated from a supply period of a driving current for a fuel injecting valve, whether or not fuel injection amount control is being normally performed, the monitoring device being characterized by comprising: injection monitoring signal outputting means configured to output the injection monitoring signal, the signal level switching from Lo to Hi or from Hi to Lo in response to a switching of the driving current between a value at which fuel injection from the fuel injecting valve is performed and a value at which the fuel injection is suspended; and a central processing unit with an input port receiving the injection monitoring signal from the injection monitoring signal outputting means, the central processing unit including: time acquiring means configured to acquire a time at which a port level, which is the signal level of the injection monitoring signal received by the input port, has switched from Lo to Hi or from Hi to Lo; port level detecting means configured to detect a port level at a certain delay after the switching from Lo to Hi or from Hi to Lo of the port level of the injection monitoring signal received by the input port; port level storing means configured to store, in response to a detection of the port level after the switching by the port level detecting means, either the port level after the switching or a flag value that is set in accordance with the port level after the switching; injection amount calculating means configured to calculate the monitored injection amount based on the time acquired by the time acquiring means and an injection period calculated from the port level detected by the port level detecting means; and monitoring means configured to monitor, by comparing the monitored injection amount calculated by the injection amount calculating means and the required fuel injection amount, whether or not the fuel injection amount control is being normally performed, the central processing unit being configured to acquire, by the time acquiring means, a time at which the port level of the injection monitoring signal has switched from Hi to Lo as a fuel injection starting time, and to acquire, by the time acquiring means, a time at which the port level of the injection monitoring signal has switched from Lo to Hi as a fuel injection ending time, so as to calculate the injection period based on the fuel injection starting time and the fuel injection ending time acquired by the time acquiring means, the central processing unit being configured to, when the port level of the injection monitoring signal received by the input port has switched from Lo to Hi or from Hi to Lo: establish the time acquired by the time acquiring means as a time to be used to calculate the injection period if the port level detected by the port level detecting means differs from either the port level currently stored in the port level storing means or the port level that is in accordance with the flag value currently stored in the port level storing means; and not use the time acquired by the time acquiring means to calculate the injection period if the port level detected by the port detecting means matches either the port level currently stored in the port level storing means or the port level that is in accordance with the flag value currently stored in the port level storing means. injection monitoring signal outputting means configured to output the injection monitoring signal, the signal level switching from Lo to Hi or from Hi to Lo in response to a switching of the driving current between a value at which fuel injection from the fuel injecting valve is performed and a value at which the fuel injection is suspended; and a central processing unit with an input port receiving the injection monitoring signal from the injection monitoring signal outputting means, time acquiring means configured to acquire a time at which a port level, which is the signal level of the injection monitoring signal received by the input port, has switched from Lo to Hi or from Hi to Lo; port level detecting means configured to detect a port level at a certain delay after the switching from Lo to Hi or from Hi to Lo of the port level of the injection monitoring signal received by the input port; port level storing means configured to store, in response to a detection of the port level after the switching by the port level detecting means, either the port level after the switching or a flag value that is set in accordance with the port level after the switching; injection amount calculating means configured to calculate the monitored injection amount based on the time acquired by the time acquiring means and an injection period calculated from the port level detected by the port level detecting means; and monitoring means configured to monitor, by comparing the monitored injection amount calculated by the injection amount calculating means and the required fuel injection amount, whether or not the fuel injection amount control is being normally performed,