Characteristics of an arch-type vortex formed behind a plate of aspect ratio h/w = 0.8 (h: height, w: width) which is placed vertically in a laminar boundary layer are investigated. Flow visualization and analysis of velocity signals yield the following new understandings. (i) The variation of Strouhal number S_t (=f_cw/U_o) with Reynolds number Re (=wU_o/v) can be classified into three regions. (ii) Arch-type vortex formed behind a plate shows inconsistency in its periodicity and strength, and has no regular mode vortex shedding as in two-dimensional circular cylinders. (iii) Different tendency of Strouhal number distribution against Reynolds number is due to the different time rate of intermittent vortex shedding. (iv) Arch-type vortex formed behind a plate starts to dissipate in the very vicinity downstream of the plate before convectim far downstream.