To provide a technique capable of promoting workability of downward partition walls stepping across a joint section in a unit method and sufficiently ensuring strength or the like after the construction.

A construction method of the downward partition walls 5 stepping across the joint section 3 between units 2 and 2 includes a first process executing respectively part of cores 51 of cores 51 and 52 of the downward partition walls 5 to both units 2 and 2 connected to each other in a plant or the like, a second process executing the remaining cores 52 of the downward partition walls 5 after the units 2 and 2 are connected to each other and a third process sticking the downward partition wall plaster boards on the cores 51 and 52 after the second process.