A unit heat value Q.sub.pass (=I.sub.pass.sup.2.multidot.t.sub.pass) per
pass is calculated from a current value (an effective current value)
I.sub.pass of a carriage motor with one pass of a carriage (one-way
running in a reciprocation) and a moving time (a motor driving time)
t.sub.pass of the carriage. Then, the unit heat value Q.sub.pass is
successively calculated to obtain a heat value Q.sub.sigma for a unit time
T.sub.box (=one minute). The heat value Q.sub.sigma is converted into a
heat generation temperature .DELTA.T.sub.new. The heat generation
temperature .DELTA.T.sub.new is successively added in consideration of a
spontaneous radiation with the passage of time to obtain a current heat
generation temperature .DELTA.T.sub.sum (.DELTA.T.sub.sum
=K.multidot..DELTA.T.sub.sum +.DELTA.T.sub.new ; K is a radiation factor).
When .DELTA.T.sub.sum exceeds a threshold, a pause time for one pass of
the carriage is provided.
