Information of plant based on color of a surface of plant is acquired from image data obtained by imaging plant, allowing to acquire information of plant at low cost, compared to chlorophyll meter or spectroscopic analyzer. In crop production like rice plant, fertilization management including fertilizer application management like fertilizer amount determination, or other agricultural works, is supported through a smart phone or the like based on data to be observed, like converted leaf color value is calculated from image data obtained by crop imaging. Camera is connected to smart phone. Converted leaf color value can be obtained from image data obtained by imaging leaf of rice plant by camera. Converted leaf color value is transmitted to management server, for example, amount information of applied fertilizer is required in case where converted leaf color value is less than standard, can be obtained as management information for fertilizer application management.