A structure of an intake manifold (1) for an engine is constituted of divided pieces (10, 20, 30). The structure of the intake manifold includes a gas supply hole (53) formed in a predetermined position of an outer surface of the intake manifold, and configured to supply predetermined gas from the outside, and a gas inlet passage (51) formed in predetermined joint surfaces (10m, 20m) of the divided pieces (10, 20), and configured to guide the gas supplied through the gas supply hole to a predetermined gas inlet position (11) of an intake passage (E1).