A diesel engine comprising: a cylinder; a piston which reciprocates in said cylinder; a cylinder head; a fuel injector ( 17; 177) which is arranged on said cylinder head and injects fuel substantially radially approximately from a center of said cylinder; a substantially circular cavity ( 30; 130) which is formed on a top of said piston; and a combustion chamber which is defined by said top of said piston including said cavity ( 30; 130), said cylinder, and a face of said cylinder head that faces said top of said piston, wherein a wall shape of said cavity ( 30; 130) on a cross section including a center axis ( X-X) of said cylinder comprises an inward protruding portion ( 32; 132) located at the periphery of an opening ( 31; 131) of said top of said piston for said cavity ( 30; 130), a center portion ( 34; 134) projecting toward said fuel injector ( 17; 117), and a peripheral portion ( 33; 133) which connects said inward protruding portion ( 32; 132) and said center portion ( 34; 134) and includes arcs having their centers on the inner side of said cavity ( 30; 130), wherein said fuel injector ( 17; 117) and said cavity ( 30; 130) are configured so that fuel injected from said fuel injector ( 17; 117) directs to a proximity of the border between said inward protruding portion ( 32; 132) and said peripheral portion ( 33; 133), wherein said peripheral portion ( 33; 133) consists of a first portion ( 33a; 133a) located farthermost from said fuel injector ( 17; 117) among the entire peripheral portion, a second portion ( 33b; 133b) located between said inward protruding portion ( 32; 132) and said first portion ( 33a; 133a), and a third portion ( 33c; 133c) located between said first portion ( 33a; 133a) and said center portion ( 34; 134), wherein said first and third portions (33a, 33c; 133a, 133c) are arc-shaped, and wherein radii of arcs of said peripheral portion ( 33; 133) increase from said first portion ( 33a; 133a) to said third portion ( 33c; 133c), characterized in that said second portion (33b; 133b) is arc-shaped and radii of arcs decrease from said second portion ( 33b; 133b) to said first portion ( 33a; 133a), said second portion ( 33b; 133b) and said third portion ( 33c; 133c) are formed substantially symmetric with respect to a line ( Y-Y) connecting a point in said first portion ( 33a; 133a) located farthermost from said fuel injector ( 17; 117) and an injection hole ( 17a; 117a) of said fuel injector ( 17; 117), and radii of arcs of the second portion (33b; 133b) and the third portion (33c; 133c) are substantially equal and the radius of the first portion (33a; 133a) is set to be smaller than the radius of the second and third portions (33b, 33c; 133b, 133c). a cylinder; a piston which reciprocates in said cylinder; a cylinder head; a fuel injector ( 17; 177) which is arranged on said cylinder head and injects fuel substantially radially approximately from a center of said cylinder; a substantially circular cavity ( 30; 130) which is formed on a top of said piston; and a combustion chamber which is defined by said top of said piston including said cavity ( 30; 130), said cylinder, and a face of said cylinder head that faces said top of said piston, wherein a wall shape of said cavity ( 30; 130) on a cross section including a center axis ( X-X) of said cylinder comprises an inward protruding portion ( 32; 132) located at the periphery of an opening ( 31; 131) of said top of said piston for said cavity ( 30; 130), a center portion ( 34; 134) projecting toward said fuel injector ( 17; 117), and a peripheral portion ( 33; 133) which connects said inward protruding portion ( 32; 132) and said center portion ( 34; 134) and includes arcs having their centers on the inner side of said cavity ( 30; 130), wherein said fuel injector ( 17; 117) and said cavity ( 30; 130) are configured so that fuel injected from said fuel injector ( 17; 117) directs to a proximity of the border between said inward protruding portion ( 32; 132) and said peripheral portion ( 33; 133), wherein said peripheral portion ( 33; 133) consists of a first portion ( 33a; 133a) located farthermost from said fuel injector ( 17; 117) among the entire peripheral portion, a second portion ( 33b; 133b) located between said inward protruding portion ( 32; 132) and said first portion ( 33a; 133a), and a third portion ( 33c; 133c) located between said first portion ( 33a; 133a) and said center portion ( 34; 134), wherein said first and third portions (33a, 33c; 133a, 133c) are arc-shaped, and wherein radii of arcs of said peripheral portion ( 33; 133) increase from said first portion ( 33a; 133a) to said third portion ( 33c; 133c), characterized in that said second portion (33b; 133b) is arc-shaped and radii of arcs decrease from said second portion ( 33b; 133b) to said first portion ( 33a; 133a), said second portion ( 33b; 133b) and said third portion ( 33c; 133c) are formed substantially symmetric with respect to a line ( Y-Y) connecting a point in said first portion ( 33a; 133a) located farthermost from said fuel injector ( 17; 117) and an injection hole ( 17a; 117a) of said fuel injector ( 17; 117), and radii of arcs of the second portion (33b; 133b) and the third portion (33c; 133c) are substantially equal and the radius of the first portion (33a; 133a) is set to be smaller than the radius of the second and third portions (33b, 33c; 133b, 133c).