To provide the safe economical execution method of a temporary cofferdam by installing and executing the temporary cofferdam in a short period at any time regardless of the water level of a dam, etc.

Pedestal concrete 2 is placed on the foundation ground 5 of a dam upstream section, and each block 1 of a temporary cofferdam having double steel shell structure consisting of partition walls 11, in which opposed surfaces to the wall surface of the dam upstream form recessed sections 4, and having bottom slabs 12 in the blocks 1 in lowermost sections is bottomed on the pedestal concrete 2 while being successively joined. Water in the recessed sections 4 is drained, and the partition walls 11 are removed.