To provide a feed-off-arm type sewing machine for flat seaming which can meet the demand for the increase in the operating speed while preventing soiling of cloth caused by leaking out of lubrication oil.

A needle bar 13 supported in a head unit 12 is connected with a needle bar lever 21 extending from the inside of a sewing machine arm 10 and the needle bar 13 is moved up and down by swing of the needle bar lever 21 about a supporting shaft 24 generated by transmission from a sewing machine main shaft 4. The sewing machine arm 10 and the head unit 12 are fluid-tightly shielded by a shield plate 5, and a first sealing part 6 is provided in the shield plate 5. The first sealing part 6 supports the needle bar lever 21 by fitting a portion of the needle bar lever 21 into seal plates 61 held between a presser plate 62 and a back face of a seal support 60 having a curved shape of a circular arc taken along a swinging locus of the needle bar lever 21. The inside of the sewing machine arm 10 is oil-lubricated, and the inside of the head unit 12 is grease-lubricated.