in an embodiment shown in fig1 , a putter head 1 is constructed by combining a face member 2 , an intermediate member 3 , and a back member 4 . the face member 2 is formed of a metal material such as an aluminum alloy ( specific gravity : 2 . 7 ) or stainless steel ( specific gravity : 7 . 8 ). the intermediate member 3 is formed of a resin material , frp , or a light metal ( such as polycarbonate , a nylon resin , carbon fiber - reinforced resin ( specific gravity : 1 . 5 ), or an aluminum alloy ( specific gravity : 2 . 7 )). the back member 4 is formed of a metal of a heaviest specific gravity ( brass ( specific gravity : 8 . 2 ), a tungsten - nickel alloy ( specific gravity : 11 . 3 ), or the like ). a hollow structure may be adopted without using the intermediate member 3 . in this example , a relationship is adopted such that the specific gravity of the intermediate member 3 & lt ; the specific gravity of the face member 2 ≦ the specific gravity of the back member 4 . it should be noted that the head width w and the depth of the center of gravity , zgr , in fig1 are lengths from a leading edge of a face surface 2 a provided with a loft angle . the face member 2 is formed of a material having a specific gravity of 2 . 7 to 8 . 0 , and in addition to the aforementioned metal materials it is also possible to use such as duralumin , titanium , a titanium alloy , and maraging steel . the intermediate member 3 is formed of a material having a specific gravity of 0 . 9 to 3 . 0 , and suitable for use as the aforementioned resin material are a nylon resin , an epoxy resin , a polycarbonate , abs , bmc ( bulk molding compound , which is preferable since its coefficient of contraction of the resin is low and its strength is high ), smc ( sheet molding compound , which has a longer fiber length than bmc and has a higher strength ), and the like . furthermore , it is possible to suitably use as the intermediate member 3 a cloth fiber - reinforced resin ( a fiber reinforced resin sheet formed into a cloth shape by weaving such as glass fibers or carbon fibers ; if used in an outer layer , the mesh pattern is visible and is beautiful in terms of the outer appearance ), a magnesium alloy ( specific gravity : 1 . 7 to 2 . 0 ), an aluminum alloy ( specific gravity : 2 . 7 to 2 . 8 ), and the like . the back member 4 is formed of a material having a specific gravity of 8 . 0 to 19 . 2 , and in addition to the aforementioned materials it is possible to suitably use such as maraging steel ( specific gravity 8 . 0 ), a copper alloy ( specific gravity 8 . 2 to 8 . 6 ) such as brass or a beryllium - copper alloy , a tungsten - copper alloy , tungsten ( specific gravity : 19 . 2 ), and the like . in fig1 , reference numeral 5 denotes a hole in which a shaft not shown ) is inserted and fixed , and reference numeral 2 a denotes the face surface . a recessed portion may be provided in a ball striking portion of this face surface 2 a , and a resin may be fitted or bonded therein . in addition , as for the intermediate portion 3 , a metal having a light specific gravity such as an aluminum alloy or a ceramic should preferably be disposed to the sole surface to ensure that the sole surface is difficult to damage . fig2 is an exploded perspective view illustrating another embodiment , in which a nylon resin - made ball striking plate 6 is fitted in a recessed portion 2 b formed in the face surface 2 a of the face member 2 . further , the intermediate member 3 located in the rear of the face member 2 is formed into an inverse u - shape with its lower side open by attaching a cloth fiber - reinforced resin sheet to the surface of bmc . an aluminum alloy - made sole portion 7 is attached to a lower side of this intermediate member 3 . furthermore , the back member 4 has its main body formed of stainless steel , and is formed by screwing a tungsten - nickel alloy - made screw 8 into this main body . fig3 shows the putter head in which the members shown in fig2 are assembled . a putter head of a type which is a so - called “ pin type ” in terms of the shape of the putter head was set as comparative example 1 (( 1 ) in the tables ); a putter head of a type which is a so - called “ mallet type ” was set as comparative example 2 (( 2 ) in the tables ); and a putter head of the “ 2 - ball type ” of callaway golf company ( odyssey white hot 2 - ball blade ) was set as comparative example 3 (( 3 ) in the tables ). various comparisons were made between these putter heads and two examples a and b (( 4 ) and ( 5 ) in the tables ) of the invention . in example a , an aluminum alloy was used as the face member , and smc ( carbon resin - reinforced resin ) was used as the intermediate member , and a copper alloy was used as the back member . example b is an example of fig2 . as a result of conducting actual ball hitting tests using putters in which the respective putter heads shown in table 1 were installed , the evaluation of the ease of swing is shown in table 2 , and the evaluation of the off - center hit performance is shown in table 3 . the off - center hit performance is based on the total evaluation of how the ball rolls and the directionality when the ball is hit off the sweet spot . in addition , the mark ⊚ in the tables shows an evaluation that it is “ very excellent ;” the mark “◯,” it is “ good ;” the mark “ δ ,” it is “ ordinary ;” and the mark “ x ,” it is “ bad .” in both examples a and b , both of the deep center of gravity and the high moment of inertia were realized , and the putter heads were of shapes which do not present a strange feeling as compared with the conventional mallet - type putter . if the center of gravity becomes deep , the ease of swing and the stability of the stroke improve , and if the moment of inertia becomes large , the smash factor , i . e ., the head speed / ball speed ratio , in off - center hits improves .
a putter head wherein a depth of a center of gravity , zgr , is a value greater than half of a head width w , namely , zgr ≧ w / 2 , and that a moment of inertia , iy , about the center of gravity is such that iy ≧ 4000 g · cm 2 .