A multi-functional sensor for deriving information on a recording sheet material comprising an impact-applying unit for applying an external mechanical force onto the recording sheet material, a signal-detecting unit for detecting a signal of response of the recording sheet material to the external mechanical force, a signal-processing unit for processing the signal, and a signal-judging unit for deriving a property of the recording sheet material by comparison of a signal from the signal-detecting unit or the signal processing unit with information memorized preliminarily, wherein the signal-processing unit comprises a separation section for separating the signal from the signal-detecting unit into output signals on the properties of the recording sheet material, and a processing section for deriving, by statistical treatment, a correlation equation showing a correlation between the separated output signal and the properties of the recording sheet material.