fig1 to 3 show an embodiment according to the first aspect of the invention , where fig1 is a sectional view of a distributor , fig2 is an explanatory view of communicating holes of the distributor , and fig3 is a sectional view of one of the holes . in fig1 reference numeral 1 denotes a cup - shaped housing which serves as a distributor body and is made through an aluminum die casting , and bearings 3 and 4 are provided in the housing so as to removably support a shaft 2 . reference numeral 5 denotes a light - emitting diode , and reference numeral 6 denotes a light - receiving diode . a rotor plate 8 which has a plurality of slits formed therein is provided in a gap 7 between the two diodes and is fixedly clamped between a rotor shaft 10 and the shaft 2 by means of a screw 9 . a distribution rotor 11 is fitted to the upper portion of the rotor shaft 10 . the distribution rotor 11 is so constructed as to be electrically connected through a gap to a side electrode 13 provided on a cap 12 . holes 20 , 30 for communicating the inside of the distributor with the outside thereof are provided in the housing 1 . the communicating hole 20 is a hole for exhaust or air outlet and opens on the upstream side of a baffle plate 14 with respect to the direction of rotation , of the rotor , as shown in fig2 . on the other hand , the communicating hole 30 is a hole for suction or air inlet and is located on the downstream side of a baffle plate 15 . the air containing ozone and nitrogen oxides in the distributor is rotated in a direction as the distribution rotor 11 rotates , which direction is denoted by an arrow r and is the same as the direction of rotation of the rotor , and strikes the baffle plates so that a negative pressure is produced on the downstream sides of the baffle plates and a positive pressure is produce on the upstream sides thereof . accordingly , the atmosphere is drawn from the communicating hole 30 , as shown by arrows in , and the air in the distributor is exhausted through the communicating hole 20 into the atmosphere as shown by arrows out . reference numerals 20a , 30a each denote a communicating passage of the communicating hole on the distributor side and reference numerals 20b , 30b each denote a communicating passage of the communicating hole on the atmosphere side . fig3 is a sectional view of the communicating hole 20 for exhaust . the communicating hole 30 for suction has a similar configuration . when the shaft 2 is rotated in synchronism with an engine , the rotor plate 8 , which is integral with the shaft 2 , is rotated in the gap 7 between the light - emitting diode 5 and the light - receiving diode 6 , and electrical signals for detecting , for instance , the number of revolutions of the engine , the piston position of the engine and the like are formed in a control circuit unit 16 by the passage of the plurality of slits provided around the periphery of the rotor plate 8 between the diodes . these electrical signals are input into a control unit ( not shown ), which comprises a microcomputer , so as to generate a high secondary voltage in a ignition coil ( not shown ) and produce a discharge from the distribution rotor 11 to the side electrode 13 . thus , electricity is supplied to a spark plug ( not shown ) to ignite an air - fuel mixture in a cylinder of the engine . the discharge between the distribution rotor 11 and the side electrode 13 requires the provision of a shield cover 17 above the control circuit unit 16 . in the embodiment having the structure described above , large amounts of ozone and nitrogen oxides are generated during operation by the discharge between the distribution rotor 11 and the side electrode 13 and , when these substances react with water , nitric acid is produced . the ozone and nitric acid will corrode the components in the distributor and hinder the operation thereof , unless the distributor is ventilated . according the above embodiment , however , ventilation is provided by the communicating hole 30 for suction and the communicating hole 20 for exhaust , as described with reference to fig2 . the resistance of air of this ventilation is small and dust does not readily stay in the bend of each communicating hole , because the angle θ of bend of each communication hole is an obtuse angle as illustrated in the case of the communicating hole 20 in fig3 . fig4 shows an embodiment according to the second aspect of the invention , and is a sectional view corresponding to fig3 of the first embodiment . in this embodiment , each communicating holes 20 &# 39 ; or 30 &# 39 ; is formed through a rubber grommet 40 which is fitted between the cap 12 and the shied cover 17 of the distributor . the communicating hole 20 &# 39 ; ( 30 &# 39 ;) of this embodiment comprises a communicating passage 20a &# 39 ; ( 30a &# 39 ;) on the distributor side and a communicating passage 20b &# 39 ; ( 30b &# 39 ;) on the atmosphere side which are connected to each other by means of a passage bent in a circular arc form with a radius of curvature r . the communication holes of this configuration can achieve a function and effect which is similar to that of the first embodiment . fig5 shows another embodiment according to the second aspect of the invention , which differs from the embodiment of fig4 . in this embodiment , two bent portions or passages each in the form of a circular arc are provided in a grommet 40 &# 39 ;, and each communicating hole has an intermediate communicating passage 20c &# 39 ; ( 30c &# 39 ;) provided between a communicating passage 20a &# 39 ; ( 30a &# 39 ;) on the distributor side and a communicating passage 20b &# 39 ; ( 30b &# 39 ;) on the atmosphere side . the communicating hole 20 &# 39 ; and 30 &# 39 ; of this configuration also can achieve a function and effect which is the same as that of the above - described embodiment shown in fig4 . additionally , the present invention can be applied also to a distributor in which a fan is provided on the lower side of a rotor 11 to ensure that an air flow is positively generated in the distributor . as described above , a distributor to which the present invention is applied exhibits excellent practical effects in that it has a good ventilation efficiency and there is no danger of it becoming clogged with dust .