The agency says declines in the petroleum and coal product, aerospace product and parts, and machinery industries were largely offset by smaller gains in most other industries.
Economists had expected a gain of 0.4%, according to Thomson Reuters.
In April, 14 of 21 industries representing about 60% of manufacturing posted gains, despite the slight decrease in total sales.
Read: Ontario on the rebound
Sales in the petroleum and coal product industry decreased 5% in April to $6.9 billion, due to partial shutdowns at several refineries for maintenance and retooling. In contrast, sales in the motor vehicle industry rose 3.3% to $4.4 billion.
Sales were down in five provinces in April, with Quebec posting the largest decrease.