an electron gun in a preferred embodiment according to the present invention as shown in fig6 for a three - gun three - beam type electron gun will be described with reference to the accompanying drawings . referring to fig6 in a three - gun three - beam type electron gun , the sectional shape of an electron beam is dependent mainly on the configuration of a second grid g2 . therefore , the present invention forms the second grid g2 , i . e ., a beam control electrode , in a configuration which will be described below . fig1 is a plan view of a second grid g2 , i . e ., a beam control electrode , in accordance with the present invention , as viewed from the side of a screen . a beam is moved along a horizontal axis x - x &# 39 ; for horizontal scanning . beam passage holes 22r , 22g and 22b of a predetermined diameter for three beams are formed in the second grid g2 at predetermined intervals on a horizontal line . a pair of excess metal relieving slots 40r and 41r , a pair of excess metal relieving slots 42g and 43g , and a pair of excess metal relieving slots 44b and 45b are formed on the opposite transverse ends of the beam passage holes 22r , 22g and 22b , respectively . the excess metal relieving slots 40r to 45b are separated from the beam passage holes to avoid the influence of the excess metal relieving slots 40r to 45b on electric fields to be applied to beams . in this embodiment , the horizontal center distance wb ( fig3 ) between each of the beam passage holes and each of the corresponding vertically elongate excess metal relieving slots 40r to 45b is about twice the diameter of the beam passage holes 22r , 22g and 22b . although there is no restriction on the width of the excess metal relieving slots 40r to 45b , the width is slightly smaller than the diameter of the beam passage holes in this embodiment . the length wc ( fig3 ) of the excess metal relieving slots 40r to 45b is about twice the diameter of the beam passage holes . the number , the positions and the sizes of the excess metal relieving slots 40r to 45b are selectively determined by taking into consideration the diameter of the beams , the ratio of the area of thin portions to that of thick portions , and the thickness of the electrode plate . the description of fig1 is only an example . rectangular thin portions 20r , 20g and 20b are formed in the electrode plate so as to include the excess metal relieving slots 40r to 45b partly . the rectangular thin portions 20r , 20g and 20b are horizontally elongate portions having their long sides parallel to the axis x - x &# 39 ;. as shown in fig1 b , the thin portions 20r , 20g and 20b are formed by press working in a thickness about 3 / 4 to 1 / 5 of the thickness of the electrode plate 18 . in this embodiment , the thickness of the thin portions 20r , 20g and 20b is 1 / 2 of that of the electrode plate 18 . when the electrode plate 18 is subjected to press working , excess metal of the electrode plate 18 is force to bulge outside a pressing punch . although the horizontal length of the thin portions increases slightly , the excess metal is relieved perfectly by the prepared hole 26r and the pairs of excess metal relieving slots 40r to 45b . therefore , although the widths of the excess metal relieving slots 40r to 45b are decreased slightly by press working , the excess metal does not rise in the thick portion and does not form any protrusion like the protrusion 24 . thus , no portion of the electrode plate 18 is caused to rise in protrusions by the excess metal when the electrode plate 18 is subjected to press working to form the thin portions . since any protrusion is formed in the electrode plate 18 , the second grid g2 and the third grid g3 can accurately be held at a predetermined interval determined by the spacer 34 for beading . therefore , an electron gun 10 having desired characteristics can be constructed . thus , the horizontally elongate thin portions 20r , 20g and 20b respectively including the beam passage holes 22r , 22g and 22b are formed by press working to obtain the second grid g2 as shown in fig1 a . the horizontally elongate beam passage holes 22r , 22g and 22b increase the divergence angle θ of the beams with respect to the horizontal scanning direction , and the beams travel through the peripheral portion of the main electron lens ml , so that the beams undergo the strongest converging action . consequently , the beam is distorted so as to form a vertically elongate spot at the center of the screen as shown in fig5 b . originally , the beam is distorted so as to form a horizontal spot in the corners of the screen as shown in fig5 a , the beam is corrected so as to form a circular spot in the corners of the screen , because the beam is distorted by a deflecting magnetic field in the periphery of the screen . when a beam is deflected for scanning by an electron gun provided with the foregoing second grid g2 , spots of substantially the same shape are formed at different positions on the screen . therefore a crt provided with this electron gun realizes a stable shape of a beam for the periphery of the screen and hence pictures can be displayed in an improved picture quality at any position on the screen . fig2 is a view of assistance in explaining a method of fabricating the foregoing second grid g2 . a prepared hole 26r and the excess metal relieving slots 40r and 41r are formed at predetermined intervals on a line in an electrode plate 18 as shown in fig2 a and 2b . fig3 a is a plan view of the electrode plate 18 in a step shown in fig2 b of the method . the electrode plate 18 is subjected to press working using a rectangular punch 28 as shown in fig2 c for forming a thin portion 20r including the excess metal relieving slots 40r and 41r as shown in fig2 d . the horizontal length wd of the thin portion 20r is increased slightly by the press working . however , any excess metal is not caused to rise in a thick portion by the horizontal expansion of the thin portion 20r ; that is , any protrusion corresponding to the protrusion 24 formed in the conventional second grid is not formed ( fig3 b ). the prepared hole 20r for the beam passage hole 22r is crushed slightly by the press working ( fig2 d and 3b ). a portion of the electrode plate 18 corresponding to the prepared hole 20r is punched with a punch 30 to form the beam passage hole 22r ( fig2 e and 3c ). thus , the second grid g2 having the thin portions is completed . thin portions 20g and 20b respectively including the beam passage holes 22g and 22b are formed by the same process and hence the description of processes for forming the thin portions 20g and 20b will be omitted . the method described with reference to fig2 forms the prepared hole 26r as shown in fig2 b . if the second grid g2 is used particularly for controlling fine beams , a beam passage hole may be formed in the last step without forming any prepared hole , because it is possible that a small prepared hole for a small beam passage hole is crushed completely by press working and there is no necessity of forming the prepared hole . although the excess metal relieving slots of the second grid in this embodiment are slots as shown in fig4 a , the excess metal relieving slots may be rectangular slots as shown in fig4 b , elliptic slots as shown in fig4 c or slots of any suitable shape provided that the excess metal relieving slots are able to relieve excess metal during metal working . the vertical length of the thin portions may be equal to the diameter of the beam passage holes as shown in fig4 d . as mentioned above , the excess metal relieving slots are separated from the corresponding beam passage hole in order that the electric field is not affected by the excess metal relieving slots . it goes without saying that some contrivance , such as increasing the size of the excess metal relieving slots 40r to 45b , must be incorporated into the invention when the thin portions 20r , 20g and 20b are formed in a thickness about 1 / 5 of that of the electrode plate 18 . although the invention has been described in terms of an embodiment which is designed to shape beams in a vertically elongate cross section , there is not any particular restriction on the shape of beams . the present invention is applicable to an electron gun other than that for a three - gun three - beam type electron gun . as is apparent from the foregoing description , according to the present invention , any protrusions are not formed in the thick portion of the beam control electrode when forming the thin portions by press working , because the beam control electrode is provided with the excess metal relieving slots in combination with the beam passage holes . therefore , the beam control electrode can be formed with a high accuracy . the beam control electrode is suitable for intentionally deforming the sectional shape of beams . the electron gun employing the beam control electrode of the present invention is capable of readily controlling beams in a desired sectional shape , so that the distortion of the sectional shape of beams in the periphery of the screen of a crt employing the electron gun can easily be corrected , which improves picture quality . since the sectional shape of beams can be corrected by the electron gun without requiring any special correcting means , the construction of the crt can be simplified accordingly . thus , the electron gun of the present invention is very suitable for use on a three - gun crt .