PURPOSE: To detect the rotating speed of an engine accurately without any hindrance to the attachment and detachment of a cowling adjoining to an electric field and/or magnetic field generating part relative to the rotating speed by arranging an electric field and/or magnetic field sensing part on the internal surface of the cowling.

CONSTITUTION: A high voltage generated by an ignition coil 19 is applied to an ignition plug 17 on every turn of the engine 14 and a high secondary current flows to a high-tension cord (electric field generating part) 18. The number of pulses of the secondary current is proportional to the rotating speed of the engine 14, so electric field variations proportional to the rotating speed of the engine 14 are generated at the circumference of the high-tension cord 18. The electric field sensing part 20 is arranged on the internal surface of the cowling 16 adjoining to the high-tension cord 18 and a speed indicator body 22 equipped with a speed display part 21 for displaying the engine rotating speed based upon the sense result is arranged on the external surface of the cowling 16.