To provide a simple and safe detecting method dangerous driving condition such as driving under the influence of alcohol and drowsy driving before driving and during driving.

It is determined that whether or not a driver is in the condition where the driving can be possible according to whether or not the driver can properly input displayed random numbers in an allowable time after the random numbers are displayed in a predetermined time before travelling, according to the presence or absence of the input signal showing that the driver can recognize the display from the driver in a predetermined allowable time after dividing a display area on an HUD into M divisions and allocating numbers from 0 to (M-1) to the respective divisions, generating the random number of 0 to (M-1) in a random period, and keeping specified signals predetermined in the divisions corresponding to the random number displayed for a predetermined time during travelling. However, when recognizing the specified signal display, the sense signal is also a display recognize signal in the case normal driving operation is carried out within the predetermined allowable time.