A management center analyzes probe information uploaded from an in-vehicle terminal mounted in each of multiple probe cars to thereby generate road traffic information allowing specification of a unit road segment in a congested state. For this purpose, when forming an image on a glossy print medium, a coagulation inhibiting liquid for inhibiting a coagulation of ink is applied to at least a part of an area on the print medium where the ink is applied. A plurality of variables from the population of candidate variables are selected as a selected set based on the variable predictiveness values. The head-slap detection circuit compares a resistance between the conductive surface and the conductive film to a threshold resistance level and reports a duration of a head-slap event upon a change in resistance between the conductive surface and the conductive film exceeding the threshold resistance level.