The amount of formaldehyde was determined in edible mushroom, Lentinus edodes (BERK.) SING, Japanese name (Shiitake). When the official steam distillation method is applied to the determination of formaldehyde in the mushroom, not only free formaldehyde but acid hydrolysed aldehyde would be detected simultaneously. The first appears to be liberated by enzymic reaction, and the second to be produced by acid hydrolysis of a certain constituent of the mushroom. In view of hygienic science, only the liberated formaldehyde would be of importance, and this could be determined by direct colorimetry with acetylacetone. The results obtained on 25 samples showed the average amount of formaldehyde to be 199 μg (100-280 μg) per gram of dried mushroom, which is about one-half of that measured by the steam distillation method.