To improve jet penetration in a main combustion chamber in order to reduce fuel consumption of an auxiliary chamber type engine and delivery smoke, form good mixture of fuel and air, and promote combustion of the main combustion chamber.

In a combustion chamber structure of a diesel engine formed in such a constitution that an auxiliary combustion chamber 2 in which a fuel injection valve 5 is faced to a cylinder head 4 is arranged and is communicated with a main combustion chamber through a plurality of injection ports, two injection ports 3 are set so as to cross extending lines of an axial line of two injection ports 3 with each other at a point D on a C flat surface of a cylinder center line B-B side in relation to the center line A-A of the auxiliary combustion chamber 2, in the C flat surface including the center line A-A of the auxiliary combustion chamber 2 and the cylinder center line B-B side. A projection part 11 is arranged which extends along a nearly C flat surface line shape from an intermediate part of an opening end of the main combustion chamber formed on a part held by two auxiliary chamber injection ports 3.