A brake booster 1 of tandem type includes a solenoid 16 within a valve body 6. The solenoid 16 is provided with a forwardly disposed holder 42 and a rearwardly disposed yoke 43, with a spool 44 held sandwiched therebetween and with a piston 56 disposed to be moveable around the inner periphery. A tubular guide 56A is formed on the front portion of the piston 56 while an annular guide 56B is formed on the rear portion of the piston 56. The tubular guide 56A is slidably fitted into the inner periphery of the holder 42 while the annular guide 56B is slidably fitted into the inner periphery of the yoke 43. When the solenoid 16 is energized to cause the piston 56 to move back and forth relative to the holder 42 and the yoke 43, this movement is guided by the both guides 56A, 56B. This arrangement permits a smooth back-and-forth movement of the piston 56 as compared with the prior art.