An image-forming apparatus contains a control portion, a thermopile sensor, a temperature sensor, a fan, a heater, a guide plate temperature detection sensor, an external air temperature detection sensor, a fixing temperature detection sensor and the color sensor. The control portion calculates estimated sheet temperature of the conveying sheet using information such as guide plate temperature, external air temperature, fixing temperature and paper weight. The control portion controls the fan or the heater to drive so that sensor temperature of the thermopile sensor approaches the estimated sheet temperature of the sheet. The control portion controls the thermopile sensor to carry out a measurement of the temperature of the sheet when the sensor temperature falls within a set range in relation to the estimated sheet temperature.