A pair of engaging projections is provided on a periphery of a knob mounting section projecting from an upper case 13. A substantially octagonal annular joint member is engaged with the engaging projections and has a pair of engaging projections provided in the direction of 90 degrees off the engaging projections so as to be relatively rockably engaged with the operation knob, whereby the operation knob is universally jointed through the joint member with the knob mounting section. The operation knob is capable of being rocked by pressing an optional position thereof in the circumferential direction, and by being pivoted on two axes meeting at right angles, properly controlled steady operation can be performed. A seal plate is placed on a concave portion of the upper case. Lift springs are projectingly formed on the seal plate at intervals of about 45 degrees in the circumferential direction. When pressing the optional position in the circumferential direction of the operation knob, any of the lift springs is compressed such that a switch element located below the lift springs is turned on or off.