A scroll saw includes a pair of spaced generally parallel upper and lower arms, a saw blade, and a saw blade fastening and tension adjusting device for securing the saw blade between the upper and lower arms. The adjusting device includes a channel, an elongated blade support, upper and lower blade holders, and an adjustment unit. The channel opens downwardly and is formed on a distal end of the upper arm. The blade support has an inner end which extends into the channel and which is connected pivotally to spaced side plates of the channel. The upper blade holder is mounted on a distal end of the blade support, while the lower blade holder is mounted on the distal end of the lower arm. The upper and lower blade holders hold removably and respectively two ends of the saw blade. The adjustment unit includes a compression spring, a connecting post and a lever. The spring is disposed between a top plate of the channel and a top surface of the blade support. The connecting post has a threaded lower end which extends through the channel and the compression spring and which is connected threadedly to the blade support. The lever has a front portion which is provided with a cam surface that abuts against the top plate of the channel and which is mounted pivotally on the connecting post.