A manhole monitoring system comprising: a manhole opening/closing sensor including a combination of an acceleration sensor and an additional temperature sensor; said manhole opening/closing sensor being attached to a rear side of a manhole lid or a lid frame portion of a manhole by suspension or adhesion and configured to periodically transmit, to a gateway via a radio wave, opening/closing state data that includes sensing information from the acceleration sensor and said additional temperature sensor; a monitoring platform configured to monitor the manhole; and a gateway configured to receive a radio wave that includes at least the opening/closing state data transmitted from the manhole opening/closing sensor and transmit the opening/closing state data to the monitoring platform, and wherein the monitoring platform is configured to: collect the opening/closing state data transmitted through the gateway, analyze only extracted data that serves as a point of analysis by judging only the movement of the acceleration sensor to determine, based on the sensing information from the acceleration sensor caused by opening the lid and using a predetermined threshold value, whether the lid of the manhole is opened or closed, and to detect an abnormality in the manhole in the form of possible fire if it is determined that the manhole is opened and the temperature in the manhole has risen, and to perform control to transmit an opening/closing determination result to a control center side, if it is determined that the manhole lid of the manhole is opened, based on the opening/closing determination result of the manhole lid. a manhole opening/closing sensor including a combination of an acceleration sensor and an additional temperature sensor; said manhole opening/closing sensor being attached to a rear side of a manhole lid or a lid frame portion of a manhole by suspension or adhesion and configured to periodically transmit, to a gateway via a radio wave, opening/closing state data that includes sensing information from the acceleration sensor and said additional temperature sensor; a monitoring platform configured to monitor the manhole; and a gateway configured to receive a radio wave that includes at least the opening/closing state data transmitted from the manhole opening/closing sensor and transmit the opening/closing state data to the monitoring platform, and wherein the monitoring platform is configured to: collect the opening/closing state data transmitted through the gateway, analyze only extracted data that serves as a point of analysis by judging only the movement of the acceleration sensor to determine, based on the sensing information from the acceleration sensor caused by opening the lid and using a predetermined threshold value, whether the lid of the manhole is opened or closed, and to detect an abnormality in the manhole in the form of possible fire if it is determined that the manhole is opened and the temperature in the manhole has risen, and to perform control to transmit an opening/closing determination result to a control center side, if it is determined that the manhole lid of the manhole is opened, based on the opening/closing determination result of the manhole lid. collect the opening/closing state data transmitted through the gateway, analyze only extracted data that serves as a point of analysis by judging only the movement of the acceleration sensor to determine, based on the sensing information from the acceleration sensor caused by opening the lid and using a predetermined threshold value, whether the lid of the manhole is opened or closed, and to detect an abnormality in the manhole in the form of possible fire if it is determined that the manhole is opened and the temperature in the manhole has risen, and to perform control to transmit an opening/closing determination result to a control center side, if it is determined that the manhole lid of the manhole is opened, based on the opening/closing determination result of the manhole lid.